Optimisation

Calculus, 2ADV C3 2025 MET2 19*

Let $A$ be a point on the line $y=x+c$ and $B$ be a point on the curve $y=\log _e(x-1)$.

The tangent to the curve at point $B$ is parallel to the line $y=x+c$.

Show that the distance $(d)$ between the points $AB$ can be expressed as
$d=\sqrt{2x^2+(2c-4)x+4+c^2} $ (2 marks)

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Determine the $x$-coordinate of point $A$, in terms of $c$, when the distance $AB$ is a minimum. (2 marks)

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a. $\text{See Worked Solutions}$

b. $d_{\text{min}}=\dfrac{2-c}{2}$

Show Worked Solution

a. $\text{Gradient of}\ \ y=x+c \ \ \Rightarrow\ \ m=1$

$g(x)=\log _e(x-1), \ g^{\prime}(x)=\dfrac{1}{x-1}$

$\text{Solve} \ \ g^{\prime}(x)=1:$

$\dfrac{1}{x-1}=1 \ \Rightarrow \ x=2$

$\text{Consider the diagram for the case when}\ \ c=0:$

$\text{Find distance \((d)$ between $B(2,0)$ and $A(x, x+c)$:}\)

$d$	$=\sqrt{(x-2)^2+(x+c)^2}$
	$=\sqrt{x^2-4x+4+x^2+2cx+c^2}$
	$=\sqrt{2x^2+(2c-4)x+4+c^2}$

b. $d^2=2x^2+(2c-4)x+4+c^2$

$\dfrac{d(d^2)}{dx}=4x+2c-4$

$\dfrac{d^2(d^2)}{dx^2}=4>0$

$\text{MIN when}\ \dfrac{d(d^2)}{dx}=0:$

$4x+2c-4=0\ \ \Rightarrow\ \ x=\dfrac{4-2c}{4}=\dfrac{2-c}{2}$

$\therefore d_{\text{min}}\ \text{occurs when}\ \ x=\dfrac{2-c}{2}$

Calculus, 2ADV C3 2025 HSC 26

A piece of wire is 100 cm long. Some of the wire is to be used to make a circle of radius $r$ cm. The remainder of the wire is used to make an equilateral triangle of side length $x$ cm.

Show that the combined area of the circle and equilateral triangle is given by
$A(x)=\dfrac{1}{4}\left(\sqrt{3} x^2+\dfrac{(100-3 x)^2}{\pi}\right)$. (2 marks)

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By considering the quadratic function in part (a), show that the maximum value of $A(x)$ occurs when all the wire is used for the circle. (3 marks)
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a. $\text{See Worked Solutions}$

b. $\text{See Worked Solutions}$

Show Worked Solution

a. $\text{Area of equilateral triangle:}$

$A_{\Delta}=\dfrac{1}{2} a b\, \sin C=\dfrac{1}{2} \times x^2 \times \sin 60^{\circ}=\dfrac{\sqrt{3} x^2}{4}$

♦ Mean mark (a) 41%.

$\text{Wire remaining to make circle}=100-3 x$

$\text {Find radius of circle:}$

$2 \pi r=100-3 x \ \Rightarrow \ r=\dfrac{100-3 x}{2 \pi}$

$\text{Area of circle: }$

$A_{\text {circ }}=\pi r^2=\pi \times\left(\dfrac{100-3 x}{2 \pi}\right)^2=\dfrac{(100-3 x)^2}{4 \pi}$

$\text{Total Area}$	$=\dfrac{\sqrt{3} x^2}{4}+\dfrac{(100-3 x)^2}{4 \pi}$
	$=\dfrac{1}{4}\left(\sqrt{3} x^2+\dfrac{(100-3 x)^2}{\pi}\right)$

b. $\text{Note strategy clue in question: “By considering the quadratic..”}$

$\text {Expanding} \ A(x):$

$A(x)$	$=\dfrac{1}{4}\left(\sqrt{3} x^2+\dfrac{10\,000}{\pi}-\dfrac{600 x}{\pi}+\dfrac{9 x^2}{\pi}\right)$
	$=\dfrac{1}{4}\left(\sqrt{3}+\dfrac{9}{\pi}\right) x^2-\dfrac{150}{\pi} x+\dfrac{2500}{\pi}$

♦♦♦ Mean mark (b) 28%.

$\text{Consider limits on} \ x:$

$3 x \leqslant 100 \ \Rightarrow \ x \in\left[0,33 \frac{1}{3}\right]$

$\text{Consider vertex of concave up parabola}\ A(x):$

$x=-\dfrac{b}{2 a}=\dfrac{150}{\pi} \ ÷ \ \dfrac{1}{2}\left(\sqrt{3}+\dfrac{9}{\pi}\right) \approx 20.8$

$\text{By symmetry of the quadratic for} \ x \in\left[0,33 \dfrac{1}{3}\right],$

$A(x)_{\text{max}} \ \text{occurs at} \ \ x=0.$

$\text{i.e. when the wire is all used in the circle.}$

Calculus, 2ADV C3 2024 MET2 18*

A trapezium has the following dimensions.

Show that the total area of the trapezium is given by
$A=2x\sqrt{100-x^2}$ (1 mark)

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Find the value of $x$ which maximises the area of the trapezium below. (3 marks)

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i. $\text{See worked solutions}$

ii. $x=5\sqrt{2}$

Show Worked Solution

i. $A=\dfrac{h}{2}(a+b)$

$\text{By Pythagoras:}\ \ h=\sqrt{100-x^2}$

$A=\dfrac{\sqrt{100-x^2}}{2} \times (x + 3x) = 2x\sqrt{100-x^2}$

ii.	$\dfrac{dA}{dx}$	$=2\sqrt{(100-x^2}-\dfrac{2x^2}{\sqrt{100-x^2}}$
		$=\dfrac{2(100-x^2)-2x^2}{\sqrt{100-x^2}}$
		$=\dfrac{4(50-x^2)}{\sqrt{100-x^2}}$

$\text{Max/Min occurs when}\ \dfrac{dA}{dx}=0:$

$\dfrac{4(50-x^2)}{\sqrt{100-x^2}}$	$=0\quad(x \neq 10)$
$50-x^2$	$=0$
($\sqrt{50}-x)(\sqrt{50}+x)$	$=0$
$\therefore\ x$	$=\sqrt{50}=5\sqrt{2}\quad (0<x<10)$

$\text{Check gradient for max using table:}$

$x$	$7$	$\sqrt{50}$	$7.2$
$A^{\prime}$	$0.56$	$0$	$-1.06$
$\text{Gradient}$	$+$	$0$	$-$

$\therefore\ x=5\sqrt{2}\ \text{maximises the area}$

Two circles have the same centre $O$. The smaller circle has radius 1 cm, while the larger circle has radius $(1 + x)$ cm. The circles enclose a region $QRST$, which is subtended by an angle $\theta$ at $O$, as shaded.

The area of $QRST$ is $A$ cm$^{2}$, where $A$ is a constant and $A \gt 0$.

Let $P$ cm be the perimeter of $QRST$.

By finding expressions for the area and perimeter of $QRST$, show that $P(x)=2x+\dfrac{2A}{x}$. (3 marks)
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Show that if the perimeter, $P(x)$, is minimised, then $\theta$ must be less than 2. (3 marks)
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a. $\text{Area}\ =\ \text{Sector}\ OQR-\text{Sector}\ OTS$

$A$	$=\dfrac{\theta}{2\pi} \times \pi (1+x)^2-\dfrac{\theta}{2\pi} \times \pi \times 1^2$
$A$	$=\dfrac{\theta}{2} (1+x)^2-\dfrac{\theta}{2}$
$A$	$=\dfrac{\theta}{2}(1+2x+x^2-1)$
$A$	$=\dfrac{\theta}{2}x(x+2)$
$\theta$	$=\dfrac{2A}{x(x+2)}$

$\text{Perimeter}\ QRST$	$=\dfrac{\theta}{2\pi} \times 2\pi(x+1)+\dfrac{\theta}{2\pi} \times 2\pi (1)+2x $
	$=\theta(x+1)+\theta+2x$
	$=\theta(x+2)+2x$
	$=\dfrac{2A}{x(x+2)}(x+2)+2x$
	$=2x+\dfrac{2A}{x}$

b. $P(x)=2x+\dfrac{2A}{x}$

$P^{′}(x) = 2-\dfrac{2A}{x^2}$

$P^{″}(x) = \dfrac{4A}{x^3} \gt0\ \ (x,A \gt 0)$

$\Rightarrow\ \text{MIN when}\ \ P^{′}(x)=0:$

$2-\dfrac{2A}{x^2}$	$=0$
$2x^2$	$=2A$
$x^2$	$=A$

$\text{Using part (a):}$

$\theta$	$=\dfrac{2A}{x(x-2)}$
	$=\dfrac{2x^2}{x(x+2)}$
	$=\dfrac{2x}{x+2}$
	$=2 \times \dfrac{x}{x+2} $
	$ \lt 2\ \ \Big(\text{since}\ \ \dfrac{x}{x+2} \lt 1 \ \ \text{for all}\ x \Big) $

$\therefore\ \text{If}\ P(x)\ \text{is minimised,}\ \theta \lt 2. $

Show Worked Solution

a. $\text{Area}\ =\ \text{Sector}\ OQR-\text{Sector}\ OTS$

$A$	$=\dfrac{\theta}{2\pi} \times \pi (1+x)^2-\dfrac{\theta}{2\pi} \times \pi \times 1^2$
$A$	$=\dfrac{\theta}{2} (1+x)^2-\dfrac{\theta}{2}$
$A$	$=\dfrac{\theta}{2}(1+2x+x^2-1)$
$A$	$=\dfrac{\theta}{2}x(x+2)$
$\theta$	$=\dfrac{2A}{x(x+2)}$

$\text{Perimeter}\ QRST$	$=\dfrac{\theta}{2\pi} \times 2\pi(x+1)+\dfrac{\theta}{2\pi} \times 2\pi (1)+2x $
	$=\theta(x+1)+\theta+2x$
	$=\theta(x+2)+2x$
	$=\dfrac{2A}{x(x+2)}(x+2)+2x$
	$=2x+\dfrac{2A}{x}$

♦ Mean mark (a) 41%.
COMMENT: Sector/arc calculations used in solution are for those who don’t want to remember formulas.

b. $P(x)=2x+\dfrac{2A}{x}$

$P^{′}(x) = 2-\dfrac{2A}{x^2}$

$P^{″}(x) = \dfrac{4A}{x^3} \gt0\ \ (x,A \gt 0)$

$\Rightarrow\ \text{MIN when}\ \ P^{′}(x)=0:$

$2-\dfrac{2A}{x^2}$	$=0$
$2x^2$	$=2A$
$x^2$	$=A$

$\text{Using part (a):}$

$\theta$	$=\dfrac{2A}{x(x-2)}$
	$=\dfrac{2x^2}{x(x+2)}$
	$=\dfrac{2x}{x+2}$
	$=2 \times \dfrac{x}{x+2} $
	$ \lt 2\ \ \Big(\text{since}\ \ \dfrac{x}{x+2} \lt 1 \ \ \text{for all}\ x \Big) $

$\therefore\ \text{If}\ P(x)\ \text{is minimised,}\ \theta \lt 2. $

♦♦♦ Mean mark (b) 12%.

Calculus, 2ADV C3 2023 HSC 24

A gardener wants to build a rectangular garden of area 50 m² against an existing wall as shown in the diagram. A concrete path of width 1 metre is to be built around the other three sides of the garden.

Let $x$ and $y$ be the dimensions, in metres, of the outer rectangle as shown.

Show that $y$ = $\dfrac {50}{x - 2}+1 $. (1 mark)

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Find the value of $x$ such that the area of the concrete path is a minimum. Show that your answer gives a minimum area. (4 marks)

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$\text{See worked solutions}$
$x=12$

Show Worked Solution

a.	$ (y-1)(x-2)$	$=50$
	$yx-2y-x+2$	$=50$
	$y(x-2)$	$=48+x$
	$y$	$=\dfrac{48+x}{x-2}$
	$y$	$=\dfrac{50+(x-2)}{x-2}$
	$y$	$=\dfrac{50}{x-2}+1$

b. $\text{Let}\ A=\ \text{area of concrete path}$

$A$	$= xy-50$
	$= x(\dfrac{50}{x-2}+1)-50$
	$=\dfrac{50x}{x-2}+x-50$

$\dfrac{dA}{dx}$	$=\dfrac{50(x-2)-50x}{(x-2)^2}+1$
	$=\dfrac{-100}{(x-2)^2}+1$

$\text{Max/min when}\ \dfrac{dA}{dx}=0$

$\dfrac{-100}{(x-2)^2}+1$	$=0$
$(x-2)^2$	$=100$
$x-2$	$=\pm 10$

$x=12\ \ (x>0)$

♦♦ Mean mark (b) 33%.

$\text{Check gradients about}\ \ x=12: $

\begin{array} {|c|c|c|c|}
\hline
\rule{0pt}{2.5ex} x \rule[-1ex]{0pt}{0pt} & 10 & 12 & 14\\
\hline
\rule{0pt}{2.5ex}\dfrac{dA}{dx} \rule[-1ex]{0pt}{0pt} & – \frac{9}{16} & 0 & \frac{11}{36}\\
\rule{0pt}{2.5ex} \rule[-1ex]{0pt}{0pt} & \text{ \ } & \text{ _ } & \text{ / } \\
\hline
\end{array}

$\therefore \text{Minimum at}\ x=12.$

Calculus, 2ADV C3 2022 HSC 31

A line passes through the point `P(1,2)` and meets the axes at `X(x, 0)` and `Y(0, y)`, where `x>1`.

Show that `y=(2x)/(x-1)`. (2 marks)

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Find the minimum value of the area of triangle `XOY`. (4 marks)

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`text{Proof (See Worked Solutions)}`
`4\ text{u}^2`

Show Worked Solution

a. `text{Show}\ \ y=(2x)/(x-1)`

`text{S}text{ince}\ \ m_(YP)=m_(PX):`

`(y-2)/(0-1)`	`=(2-0)/(1-x)`
`y-2`	`=(-2)/(1-x)`
`y`	`=2-2/(1-x)`
	`=(2(1-x)-2)/(1-x)`
	`=(-2x)/(1-x)`
	`=(2x)/(x-1)\ \ text{… as required}`

♦♦♦ Mean mark part (a) 17%.
COMMENT: `y=(2x)/(x-1)` is the expression of a relationship between the intercepts and not the equation of the line.

b.	`A`	`=1/2 xx b xxh`
		`=1/2x((2x)/(x-1))`
		`=(x^2)/(x-1)`

`(dA)/dx`	`=((x-1)*2x-x^2(1))/((x-1)^2)`
	`=(2x^2-2x-x^2)/((x-1)^2)`
	`=(x(x-2))/((x-1)^2)`

`text{SP’s occur when}\ \ (dA)/dx=0:`

`x=0\ \ text{or}\ \ 2`

`text{Use 1st derivative test to find max/min:}`

`=>\ text{MIN at}\ \ x=2`

`:.A_min`	`=1/2 xx 2 xx (2xx2)/(2-1)`
	`=4\ text{u}^2`

♦♦ Mean mark part (b) 29%.

Calculus, 2ADV C3 2020 HSC 25

A landscape gardener wants to build a garden in the shape of a rectangle attached to a quarter-circle. Let `x` and `y` be the dimensions of the rectangle in metres, as shown in the diagram.

The garden bed is required to have an area of 36 m² and to have a perimeter which is as small as possible. Let `P` metres be the perimeter of the garden bed.

Show that `P = 2x + 72/x`. (3 marks)

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Find the smallest possible perimeter of the garden bed, showing why this is the minimum perimeter. (4 marks)

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`text(See Worked Solutions)`
`24\ text(m)`

Show Worked Solution

a.	`text(Area)`	`= xy + 1/4 pir^2`
	`36`	`= xy + 1/4 pix^2`
	`xy`	`= 36 – (pix^2)/4`
	`y`	`= 36/x – (pix)/4`

♦ Mean mark part (a) 47%.

`:. P`	`= 2x + 2y + 1/4(2pix)`
	`= 2x + 2(36/x – (pix)/4) + (pix)/2`
	`= 2x + 72/x – (pix)/2 + (pix)/2`
	`= 2x + 72/x`

b. `P = 2x + 72/x`

`(dP)/(dx)`	`= 2 – 72x^(−2)`
`(d^2P)/(dx^2)`	`= 144x^(−3)`

`text(Max or min when)\ \ (dP)/(dx) = 0:`

`2 – 72/(x^2)`	`= 0`
`2x^2`	`= 72`
`x^2`	`= 36`
`x`	`= 6,\ \ (x > 0)`

`text(When)\ \ x = 6,`

`(d^2P)/(dx^2) = 144/(6^3) = 2/3 > 0`

`=>\ text(MIN at)\ x = 6`

`:. P_text(min)`	`= 2 xx 6 + 72/6`
	`= 24\ text(m)`

Calculus, 2ADV C4 2019 HSC 16c

The diagram shows the region `R`, bounded by the curve `y = x^r`, where `r >= 1`, the `x`-axis and the tangent to the curve at the point `(1, 1)`.

Show that the tangent to the curve at `(1, 1)` meets the `x`-axis at

`qquad ((r - 1)/r, 0)`. (2 marks)

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Using the result of part (i), or otherwise, show that the area of the region `R` is

`qquad (r - 1)/(2r (r + 1))`. (2 marks)

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Find the exact value of `r` for which the area of `R` is a maximum. (3 marks)

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`text(Proof)\ text{(See Worked Solutions)}`
`text(Proof)\ text{(See Worked Solutions)}`
`r = 1 + sqrt 2`

Show Worked Solution

i.	`y`	`= x^r`
	`(dy)/(dx)`	`= r x^(r – 1)`

`text(When)\ \ x = 1, \ (dy)/(dx) = r`

♦♦ Mean mark part (i) 31%.

`text(Equation of tangent:)`

`y – 1`	`= r(x – 1)`
`y`	`= rx – r + 1`

`text(When)\ \ y = 0:`

`rx – r + 1`	`= 0`
`rx`	`= r – 1`
`x`	`= (r – 1)/r`

`:.\ text(T)text(angent meets)\ x text(-axis at)\ \ ((r – 1)/r, 0)`

ii. `text(Area under curve)`

♦♦♦ Mean mark part (ii) 21%.

`= int_0^1 x^r`

`= [1/(r + 1) ⋅ x^(r + 1)]_0^1`

`= 1/(r + 1) xx 1^(r + 1) – 0`

`= 1/(r + 1)`

`text(Area under tangent)`

`= 1/2 xx b xx h`

`= 1/2 (1 – (r – 1)/r) xx 1`

`= 1/2 (1 – (r – 1)/r)`

`:. R`	`= 1/(r + 1) – 1/2(1 – (r – 1)/r)“
	`= 1/(r + 1) – 1/(2r) [r – (r – 1)]`
	`= 1/(r + 1) – 1/(2r)`
	`= (2r – (r + 1))/(2r(r + 1))`
	`= (r – 1)/(2r(r + 1))`

iii.	`R`	`= (r – 1)/(2r(r + 1)) = (r – 1)/(2r^2 + 2r)`
	`(dR)/(dr)`	`= ((2r^2 + 2r) xx 1 – (r – 1)(4r + 2))/(2r^2 + 2r)^2`
		`= (2r^2 + 2r – 4r^2 – 2r + 4r + 2)/(2r^2 + 2r)^2`
		`= (-2r^2 + 4r + 2)/(2r^2 + 2r)^2`
		`= (-2(r^2 – 2r – 1))/(2r^2 + 2r)^2`

`text(Find)\ \ r\ \ text(when)\ \ (dR)/(dr) = 0:`

♦♦ Mean mark part (iii) 23%.

`r^2 – 2r – 1 = 0`

`r`	`= (2 +- sqrt((-2)^2 – 4 xx 1 xx (-1)))/2 `
	`= (2 +- sqrt 8)/2`
	`= 1 + sqrt 2\ \ (r >= 1)`

	`qquadr qquad`	`qquad 1 qquad`	`\ \ 1 + sqrt 2\ \ `	`qquad 3 qquad`
	`(dR)/(dr)`	`1/4`	`0`	`-1/144`

`:. R_text(max)\ text(occurs when)\ \ r = 1 + sqrt 2`

Calculus, 2ADV C3 2019 HSC 15c

The entry points, `R` and `Q`, to a national park can be reached via two straight access roads. The access roads meet the national park boundaries at right angles. The corner, `P`, of the national park is 8 km from `R` and 1 km from `Q`. The boundaries of the national park form a right angle at `P`.

A new straight road is to be built joining these roads and passing through `P`.

Points `A` and `B` on the access roads are to be chosen to minimise the distance, `D` km, from `A` to `B` along the new road.

Let the distance `QA` be `x` km.

Show that `D^2 = (x + 8)^2 + (8/x + 1)^2`. (3 marks)

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Show that `x = 2` gives the minimum value of `D^2`. (3 marks)

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`text(Proof)\ text{(See Worked Solutions)}`
`text(Proof)\ text{(See Worked Solutions)}`

Show Worked Solution

i.	`/_ BRP`	`= /_ PQA = 90^@\ \ text{(given)}`
	`/_ BPR`	`= /_ PAQ\ \ text{(corresponding)}`
	`=> Delta BRP\ text(\|\|\|)\ Delta PQA\ \ text{(equiangular)}`

`(BR)/(RP)`	`= (PQ)/(QA)`	`text{(corresponding sides of}` `text{similar triangles)}`
`(BR)/8`	`= 1/x`
`BR`	`= 8/x`

`text(Using Pythagoras,)`

♦ Mean mark part (i) 41%.

`D^2`	`= (QA + RP)^2 + (BR + PQ)^2`
	`= (x + 8)^2 + (8/x + 1)^2`

ii.	`(D^2)^{′}`	`= 2*(x + 8) + 2(-1) ⋅ 8/x^2 (8/x + 1)`
		`= 2x + 16-16/x^2 (8/x + 1)`
		`= 2x + 16-128/x^3-16/x^2`
	`(D^2)^{″}`	`= 2-(-3) ⋅ 128/x^4-(-2) ⋅ 16/x^3`
		`= 2 + 384/x^4 + 32/x^3`

`text(When)\ \ x = 2,`

♦♦ Mean mark (ii) 31%.

`(D^2)^{′} = 2 xx 2 + 16-128/8-16/4 = 0`

`=>\ text(S.P. at)\ \ x = 2`

`(D^2)^{″} = 2 + 384/16 + 32/8 > 0`

`:.\ text(MIN value of)\ D^2\ text(at)\ \ x = 2.`

Trigonometry, 2ADV T3 SM-Bank 16

Sammy visits a giant Ferris wheel. Sammy enters a capsule on the Ferris wheel from a platform above the ground. The Ferris wheel is rotating anticlockwise. The capsule is attached to the Ferris wheel at point `P`. The height of `P` above the ground, `h`, is modelled by `h(t) = 65 - 55cos((pit)/15)`, where `t` is the time in minutes after Sammy enters the capsule and `h` is measured in metres.

Sammy exits the capsule after one complete rotation of the Ferris wheel.

State the minimum and maximum heights of `P` above the ground. (1 mark)

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For how much time is Sammy in the capsule? (1 mark)

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Find the rate of change of `h` with respect to `t` and, hence, state the value of `t` at which the rate of change of `h` is at its maximum. (2 marks)

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a. `h_text(min) = 10\ text(m), h_text(max) = 120\ text(m)`

b. `30\ text(min)`

c. `t = 7.5`

Show Worked Solution

a.	`h_text(min)`	`= 65 – 55`	`h_text(max)`	`= 65 + 55`
		`= 10\ text(m)`		`= 120\ text(m)`

b. `text(Period) = (2pi)/(pi/15) = 30\ text(min)`

c.	`h′(t)`	`=65 – 55cos((pit)/15)`
	`h′(t)`	`=pi/15 xx 55sin(pi/15 t)`
		`= (11pi)/3\ sin(pi/15 t)`

`text(S)text(ince)\ \ sin(pi/15 t)_text(max) = sin (pi/2),`

`:. h′(t)_text(max)\ \ text(occurs when)`

`(pi t)/15`	`=pi/2`
`:. t`	`=pi/2 xx 15/pi`
	`=15/2\ text(minutes)\ \ (0<=t<=30)`

Calculus, 2ADV C3 2018 HSC 16a

A sector with radius 10 cm and angle `theta` is used to form the curved surface of a cone with base radius `x` cm, as shown in the diagram.

The volume of a cone of radius `r` and height `h` is given by `V = 1/3 pi r^2 h`.

Show that the volume, `V` cm³, of the cone described above is given by

`V = 1/3 pi x^2 sqrt(100-x^2)`. (1 mark)

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Show that `(dV)/(dx) = (pi x (200-3x^2))/(3 sqrt(100-x^2))`. (2 marks)

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Find the exact value of `theta` for which `V` is a maximum. (3 marks)

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`text(Proof)\ \ text{(See Worked Solutions)}`
`text(Proof)\ \ text{(See Worked Solutions)}`
`(2 sqrt 2 pi)/sqrt 3`

Show Worked Solution

i. `V = 1/3 pi r^2 h`

`text(Using Pythagoras,)`

`h = sqrt(100-x^2)`

`r = x`

`:.\ text(Volume) = 1/3 pi x^2 sqrt(100-x^2)`

♦ Mean mark (ii) 45%.

ii.	`V`	`= 1/3 pi x^2 sqrt(100-x^2)`
	`(dV)/(dh)`	`= 1/3 pi [2x ⋅ sqrt(100-x^2)-2x ⋅ 1/2 (100-x^2)^(-1/2) ⋅ x^2]`
		`= 1/3 pi [(2x (100-x^2)-x^3)/sqrt(100-x^2)]`
		`= 1/3 pi [(200 x-2x^3-x^3)/sqrt(100-x^2)]`
		`= (pi x(200-3x^2))/(3 sqrt (100-x^2))\ \ text{.. as required}`

iii. `text(Find)\ \ x\ \ text(when)\ \ (dV)/(dx) = 0`

♦♦ Mean mark (iii) 23%.

`200-3x^2`	`= 0`
`x`	`= sqrt(200/3)`

`text(When)\ \ x < sqrt(200/3),\ (200-3x^2) > 0`

`=> (dV)/(dx) > 0`

`text(When)\ \ x > sqrt(200/3),\ (200-3x^2) < 0`

`=> (dV)/(dx) < 0`

`:.\ text(MAX when)\ \ x = sqrt(200/3)`

`text(Equating the arc length of the section)`

`text(to the circumference of the cone:)`

`2 pi r ⋅ theta/(2 pi)`	`= 2 pi ⋅ x`
`10 theta`	`= 2 pi sqrt (200/3)`
`:. theta`	`= (2 pi ⋅ 10 sqrt 2)/(10 ⋅ sqrt 3)`
	`= (2 sqrt 2 pi)/sqrt 3`

Calculus, 2ADV C3 SM-Bank 13

The figure shown represents a wire frame where `ABCE` is a convex quadrilateral. The point `D` is on line segment `EC` with `AB = ED = 2\ text(cm)` and `BC = a\ text(cm)`, where `a` is a positive constant.

`/_ BAE = /_ CEA = pi/2`

Let `/_ CBD = theta` where `0 < theta < pi/2.`

Find `BD` and `CD` in terms of `a` and `theta`. (2 marks)

--- 5 WORK AREA LINES (style=lined) ---
Find the length, `L` cm, of the wire in the frame, including length `BD`, in terms of `a` and `theta`. (1 mark)

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Find `(dL)/(d theta)`, and hence show that `(dL)/(d theta) = 0` when `BD = 2CD`. (2 marks)

--- 5 WORK AREA LINES (style=lined) ---
Find the maximum value of `L` if `a = 3 sqrt 5`. (1 mark)

--- 2 WORK AREA LINES (style=lined) ---

Show Answers Only

a. `BD = a cos theta,\ \ \ CD = a sin theta`

b. `L = 4 + a + 2 a cos theta + a sin theta`

c. `text(Proof)\ text{(See Worked Solutions)}`

d. `L_max = 19 + 3 sqrt 5`

Show Worked Solution

a. `text(In)\ \ Delta BCD,`

`cos theta`	`= (BD)/a`
`:. BD`	`= a cos theta`
`sin theta`	`= (CD)/a`
`:. CD`	`= a sin theta`

b.	`L`	`= 4 + 2 BD + CD + a`
		`= 4 + 2a cos theta + a sin theta + a`
		`= 4 + a + 2a cos theta + a sin theta`

c. `text(Noting that)\ a\ text(is a constant:)`

♦ Mean mark (Vic) 35%.

`(dL)/(d theta)= – 2 a sin theta + a cos theta`

`text(When)\ \ (dL)/(d theta) = 0`,

`- 2 a sin theta+ a cos theta`	`= 0`
`a cos theta`	`= 2 a sin theta`
`:. BD`	`= 2CD\ \ text{(using part (a))}`

d. `text(SP’s when)\ \ (dL)/(d theta)=0,`

♦♦♦ Mean mark (Vic) 5%.

`- 2 a sin theta+ a cos theta`	`= 0`
`sin theta`	`=1/2 cos theta`
`tan theta`	`=1/2`

`text(If)\ \ tan theta=1/2,\ \ cos theta = 2/sqrt5,\ \ sin theta = 1/sqrt5`

`L_(max)`	`= 4 + a + 2a cos theta + a sin theta`
	`= 4 + (3 sqrt 5) + 2 (3 sqrt 5) (2/sqrt 5) + (3 sqrt 5) (1/sqrt 5)`
	`= 4 + 3 sqrt 5 + 12 + 3`
	`= 19 + 3 sqrt 5\ text(cm)`

Calculus, 2ADV C4 EQ-Bank 27

Let `f(x) = 2e^(-x/5)\ \ \ text(for)\ \ x>=0`

A right-angled triangle `OQP` has vertex `O` at the origin, vertex `Q` on the `x`-axis and vertex `P` on the graph of `f(x)`, as shown. The coordinates of `P` are `(x, f(x)).`

Find the area, `A`, of the triangle `OPQ` in terms of `x`. (1 mark)

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Find the maximum area of triangle `OQP` and the value of `x` for which the maximum occurs. (3 marks)

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Let `S` be the point on the graph of `f(x)` on the `y`-axis and let `T` be the point on the graph of `f(x)` with the `y`-coordinate `1/2`.Find the area of the region bounded by the graph of `f(x)` and the line segment `ST`. (2 marks)

--- 5 WORK AREA LINES (style=lined) ---

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a. `x e^(-x/5)`

b. `5/e\ text(u)^2`

c. `25/4 log_e (4)-15/2\ text(u²)`

Show Worked Solution

a. `text(Area)= 1/2 xx b xx h= 1/2x(2e^(-x/5))= xe^(-x/5)`

b. `text(Stationary point when)\ \ (dA)/(dx) = 0:`

♦ Mean mark (Vic) 35%.

`x(-1/5 e^(-x/5)) + e^(-x/5)`	`= 0`
`e^(-x/5)(1-x/5)`	`= 0`

`:. x= 5\ \ \ \ (e^(-x/5) >0,\ \ text(for all)\ x)`

`text(When)\ \ x = 5,\ \ A= xe^(-x/5)= 5e^-1`

`:. A_max = 5/e\ text(u², when)\ \ x = 5`

c. `text(Find)\ S:`

`F(0) = 2\ \ =>\ \ S(0, 2)`

♦♦ Mean mark (Vic) 32%.

`text(Find)\ T:\ \ \ `	`2e^(-x/5)`	`= 1/2`
	`e^(-x/5)`	`= 1/4`
	`-x/5`	`= log_e (1/4)`
	`x`	`= 5 log_e (4)`

`=> T(5log_e(4), 1/2)`

`:.\ text(Area)`	`= text(Area)\ SOAT-int_0^(5 log_e(4)) (2e^(-x/5)) dx`
	`=1/2h(a+b) + 10 [e^(-x/5)]_0^(5 log_e (4))`
	`= 5/2 log_e (4) (2 + 1/2) + 10 [e^(-log_e (4))-e^0]`
	`= 25/4 log_e (4) +10 (1/4-1)`
	`= 25/4 log_e (4)-15/2\ text(u)^2`

Calculus, 2ADV C3 2017 HSC 16a

John’s home is at point `A` and his school is at point `B`. A straight river runs nearby.

The point on the river closest to `A` is point `C`, which is 5 km from `A`.

The point on the river closest to `B` is point `D`, which is 7 km from `B`.

The distance from `C` to `D` is 9 km.

To get some exercise, John cycles from home directly to point `E` on the river, `x` km from `C`, before cycling directly to school at `B`, as shown in the diagram.

The total distance John cycles from home to school is `L` km.

Show that `L = sqrt (x^2 + 25) + sqrt (49 + (9-x)^2)`. (1 mark)

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Show that if `(dL)/(dx) = 0`, then `sin alpha = sin beta`. (3 marks)

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Find the value of `x` that makes `sin alpha = sin beta`. (2 marks)

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Explain why this value of `x` gives a minimum for `L`. (1 mark)

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`text(Proof)\ \ text{(See Worked Solutions)}`
`text(Proof)\ \ text{(See Worked Solutions)}`
`45/12`
`text(See Worked Solutions)`

Show Worked Solution

`text(Using Pythagoras:)`

`L`	`= AE + EB`
	`= sqrt (5^2 + x^2) + sqrt (7^2 + (9-x)^2)`
	`= sqrt(25 + x^2) + sqrt (49 + (9-x)^2)\ text(… as required)`

ii. `text(From diagram):`

♦ Mean mark 41%.

`sin alpha = x/sqrt(25 + x^2) and sin beta = (9-x)/sqrt(49 + (9-x)^2)`

`L`	`= sqrt(25 + x^2) + sqrt (49 + (9-x)^2)`
`(dL)/(dx)`	`= (2x)/sqrt(25 + x^2)-(2(9-x))/sqrt(49 + (9-x)^2)`

`text(If)\ (dL)/(dx) = 0:`

`(2x)/sqrt(25 + x^2)`	`= (2(9-x))/sqrt(49 + (9-x)^2)`
`x/sqrt(25 + x^2)`	`= (9-x)/sqrt(49 + (9-x)^2)`
`sin alpha`	`= sin beta\ text(… as required)`

iii. `text(If)\ sin alpha = sin beta,\ text(then)\ alpha = beta and`

♦♦ Mean mark 29%.

`Delta ACE\ text(|||)\ Delta BDE`

`text(Using corresponding sides of similar triangles:)`

`x/5`	`= (9-x)/7`
`7x`	`= 45-5x`
`12x`	`= 45`
`:. x`	`= 45/12\ text(km)`

♦♦♦ Mean mark (iv) 9%.

iv. `text(If point)\ B\ text(is reflected across the river),\ AEB\ text(will be a)`

`text(straight line.)`

`text(If any other point is chosen,)\ AEB\ text(would not be straight)`

`text(and the distance would be longer.)`

Calculus, 2ADV C3 2016 HSC 14c

A farmer wishes to make a rectangular enclosure of area 720 m². She uses an existing straight boundary as one side of the enclosure. She uses wire fencing for the remaining three sides and also to divide the enclosure into four equal rectangular areas of width `x` m as shown.

Show that the total length, `l` m, of the wire fencing is given by

`l = 5x + 720/x`. (1 mark)

--- 2 WORK AREA LINES (style=lined) ---
Find the minimum length of wire fencing required, showing why this is the minimum length. (3 marks)

--- 6 WORK AREA LINES (style=lined) ---

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a. `text(Proof)\ \ text{(See Worked Solutions)}`

b. `120\ text(m)`

Show Worked Solution

a.	`text(Area)`	`= xy`
	`720`	`= xy`
	`y`	`= 720/x`

`l`	`= 5x + y`
	`= 5x + 720/x\ \ text(… as required)`

ii.	`(dl)/(dx)`	`= 5 – 720/x^2`
	`(d^2l)/(dx^2)`	`= 1440/x^3`

`text(Max/Min when)\ \ (dl)/(dx) = 0,`

`5`	`= 720/x^2`
`x^2`	`= 144`
`x`	`= 12,\ \ x > 0`

`text(When)\ \ x = 12,\ \ (d^2l)/(dx^2) > 0`

`:.\ text(Minimum occurs when)\ \ x = 12`

`:.\ text(Minimum fencing)`

`= 5 xx 12 + 720/12`

`= 120\ text(m)`

Calculus, 2ADV C3 2004 HSC 10b

The diagram shows a triangular piece of land `ABC` with dimensions `AB = c` metres, `AC = b` metres and `BC = a` metres, where `a ≤ b ≤ c`.

The owner of the land wants to build a straight fence to divide the land into two pieces of equal area. Let `S` and `T` be points on `AB` and `AC` respectively so that `ST` divides the land into two pieces of equal area.

Let `AS = x` metres, `AT = y` metres and `ST = z` metres.

Show that `xy = 1/2 bc`. (1 mark)

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Use the cosine rule in triangle `AST` to show that

`z^2 = x^2 + (b^2c^2)/(4x^2) − bc cos A.` (2 marks)

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Show that the value of `z^2` in the equation in part (ii) is a minimum when

`x = sqrt((bc)/2)`. (4 marks)

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Show that the minimum length of the fence is `sqrt(((P − 2b)(P − 2c))/2)` metres, where `P = a + b + c`.

(You may assume that the value of `x` given in part (iii) is feasible.) (2 marks)

--- 5 WORK AREA LINES (style=lined) ---

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a. `text(Proof)\ \ text{(See Worked Solutions)}`

b. `text(Proof)\ \ text{(See Worked Solutions)}`

c. `text(Proof)\ \ text{(See Worked Solutions)}`

d. `text(Proof)\ \ text{(See Worked Solutions)}`

Show Worked Solution

a.	`text(Area)\ \ ΔABC`	`= 1/2\ bc sin A`
	`text(Area)\ \ ΔAST`	`= 1/2\ xy sin A`

`text(Area)\ \ ΔAST`	`= 1/2 xx text(Area)\ \ ΔABC\ \ \ text{(given)}`
`1/2\ xy sin A`	`= 1/2 xx 1/2\ bc sin A`
`xy sin A`	`= 1/2\ bc sin A`
`:. xy`	`= 1/2\ bc\ \ …\ text(as required.)`

b. `text(Using the cosine rule in)\ \ ΔAST,`

`z^2 = x^2 + y^2 − 2xy cos A`

`text(S)text(ince)\ \ xy`	`= 1/2\ bc`
`:. y`	`= (bc)/(2x)`

`:. z^2`	`= x^2 + ((bc)/(2x))^2 − 2x((bc)/(2x)) cos A`
	`= x^2 + (b^2c^2)/(4x^2) − bc cos A`

c.	`z^2`	`= x^2 + ((b^2c^2)/4)x^(−2) − bc cos A`
	`(d(z^2))/(dx)`	`= 2x − (2b^2c^2)/4 x^(−3)= 2x − (b^2c^2)/(2x^3)`
	`(d^2(z^2))/(dx^2)`	`= 2 + (3b^2c^2)/2 x^(−4)= 2 + (3b^2c^2)/(2x^4)`

`text(SP’s occur when)\ \ (d(z^2))/(dx)=0`

`2x − (b^2c^2)/(2x^3)`	`= 0`
`4x^4 − b^2c^2`	`= 0`
`4x^4`	`= b^2c^2`
`x^4`	`= (b^2c^2)/4`
`x^2`	`= (bc)/2`
`x`	`= sqrt((bc)/2),\ \ \ (x > 0)`

` (d^2(z^2))/(dx^2)=2 + (3b^2c^2)/(2x^4)>0\ \ \ text{(for all}\ xtext{)}`

`:.\ text(A minimum value of)\ z^2\ text(when)\ x = sqrt((bc)/2).`

d. `cos A = (b^2 + c^2 − a^2)/(2bc)`

`text(When)\ \ x = sqrt((bc)/2),`

`:. z^2`	`= x^2 + (b^2c^2)/(4x^2) − bc cos A`
	`= (bc)/2 + (b^2c^2)/(4((bc)/2)) − bc((b^2 + c^2 − a^2)/(2bc))`
	`= (bc)/2 + (bc)/2 − ((b^2 + c^2 − a^2)/2)`
	`= (2bc – b^2 − c^2 +a^2)/2`
	`= (a^2 − (b^2 − 2bc + c^2))/2`
	`= 1/2[a^2 − (b − c)^2]`
	`= 1/2[(a − (b − c))(a + (b − c))]`
	`= 1/2[(a − b + c)(a + b − c)]`
	` = 1/2[(a + b + c − 2b)(a + b + c − 2c)]`
	`= ((P − 2b)(P − 2c))/2\ \ \ text{(using}\ \ P = a + b + c text{)}`

`:.z = sqrt(((P − 2b)(P − 2c))/2)\ \ text(metres)\ \ …\ text(as required.)`

Calculus, 2ADV C3 2007 HSC 10b

The noise level, `N`, at a distance `d` metres from a single sound source of loudness `L` is given by the formula

`N = L/d^2.`

Two sound sources, of loudness `L_1` and `L_2` are placed `m` metres apart.

The point `P` lies on the line between the sound sources and is `x` metres from the sound source with loudness `L_1.`

Write down a formula for the sum of the noise levels at `P` in terms of `x`. (1 mark)

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There is a point on the line between the sound sources where the sum of the noise levels is a minimum.

Find an expression for `x` in terms of `m`, `L_1` and `L_2` if `P` is chosen to be this point. (4 marks)

--- 8 WORK AREA LINES (style=lined) ---

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a. `N = L_1/x^2 + L_2/(m-x)^2`

b. `x = (root 3 L_1\ m)/{(root 3 L_2 + root 3 L_1)}`

Show Worked Solution

`N = L/d^2`

`text(Noise from)\ L_1`	`= L_1/x^2`
`text(Noise from)\ L_2`	`= L_2/(m-x)^2`
`:. N`	`= L_1/x^2 + L_2/(m-x)^2`

b. `N = L_1\ x^-2 + L_2 (m – x)^-2`

`(dN)/(dx)`	`= -2 L_1 x^-3 + -2 L_2 (m – x)^-3 xx d/(dx) (m – x)`
	`= (-2 L_1)/x^3 + (2 L_2)/(m – x)^3`

`text(Max or min when)\ (dN)/(dx) = 0`

`(2 L_1)/x^3`	`= (2 L_2)/(m – x)^3`
`2 L_1 (m – x)^3`	`= 2 L_2\ x^3`
`L_1 (m – x)^3`	`= L_2\ x^3`
`root 3 L_1 (m – x)`	`= root 3 L_2\ x`

`root 3 L_1\ m – root 3 L_1\ x`	`= root 3 L_2\ x`
`root 3 L_2\ x + root 3 L_1\ x`	`= root 3 L_1\ m`
`x (root 3 L_2 + root 3 L_1)`	`= root 3 L_1\ m`
`x`	`= (root 3 L_1\ m)/{(root 3 L_2 + root 3 L_1)}`

`(dN)/(dx)`	`= -2 L_1\ x^-3 + 2 L_2 (m – x)^-3`
`(d^2N)/(dx^2)`	`= 6 L_1\ x^-4 – 6 L_2 (m – x)^-4 xx -1`
	`= (6 L_1)/x^4 + (6 L_2)/(m – x)^4 > 0`

`:.\ text(A minimum occurs when)`

`x = (root 3 L_1\ m)/{(root 3 L_2 + root 3 L_1)}`

Calculus, 2ADV C3 2015 HSC 16c

The diagram shows a cylinder of radius `x` and height `y` inscribed in a cone of radius `R` and height `H`, where `R` and `H` are constants.

The volume of a cone of radius `r` and height `h` is `1/3 pi r^2 h.`

The volume of a cylinder of radius `r` and height `h` is `pi r^2 h.`

Show that the volume, `V`, of the cylinder can be written as

`V = H/R pi x^2 (R-x).` (3 marks)

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By considering the inscribed cylinder of maximum volume, show that the volume of any inscribed cylinder does not exceed `4/9` of the volume of the cone. (4 marks)

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a. `text(Proof)\ \ text{(See Worked Solutions)}`

b. `text(Proof)\ \ text{(See Worked Solutions)}`

Show Worked Solution

a. `text(Show)\ \ V = H/R pi x^2 (R-x)`

♦♦ Mean mark (i) 16%.

`text(Consider)\ \ Delta ABC and Delta DEC`

`/_ ABC = /_DEC = 90°`

`/_ BCA\ \ text(is common)`

`:. Delta ABC\ \ text(|||)\ \ Delta DEC\ \ text{(equiangular)}`

`:.\ (DE)/(EC)`

`= (AB)/(BC)`

`\ text{(corresponding sides of}`

`\ text{similar triangles)}`

`y/(R-x)`

`= H/R`

`y`

`= H/R (R-x)`

`text(Volume of cylinder)`

`= pi r^2 h`

`= pi x^2 xx H/R (R-x)`

`= H/R pi x^2 (R-x)\ \ text(… as required.)`

♦♦ Mean mark (ii) 21%.

b. `V= H/R pi x^2 (R-x)`

`(dV)/(dx)`	`= H/R pi [(x^2 xx -1) + 2x (R-x)]`
	`= H/R pi [-x^2 + 2xR-2x^2]`
	`= H/R pi [2xR-3x^2]`
`(dV^2)/(dx^2)`	`= H/R pi [2R-6x]`

`text(Max or min when)\ \ (dV)/(dx) = 0`

`H/R pi [2xR-3x^2]`	`= 0`
`2xR-3x^2`	`= 0`
`x (2R-3x)`	`= 0`

`x = 0\ \ or\ \ 3x`	`= 2R`
`x`	`= (2R)/3`

`text(When)\ \ x = 0:`

`(d^2V)/(dx^2) = H/R pi [2R-0] > 0\ \ =>\ text(MIN)`

`text(When)\ \ x = (2R)/3:`

`(d^2V)/(dx^2)`	`= H/R pi [2R-6 xx (2R)/3]`
	`= H/R pi [-2R] < 0\ \ =>\ \text{MAX}`

`text(Maximum Volume of cylinder)`

`= H/R pi ((2R)/3)^2 (R-(2R)/3)`

`= H/R pi xx (4R^2)/9 xx R/3`

`= (4 pi H R^2)/27`

`text(Volume of cone)\ = 1/3 pi R^2 H`

`:.\ text(Max Volume of Cylinder)/text(Volume of cone)`

`= ((4 pi H R^2)/27)/(1/3 pi R^2 H)`

`= (4 pi H R^2)/27 xx 3/(pi R^2 H)`

`= 4/9`

`:.\ text(The volume of the inscribed cylinder does)`

`text(not exceed)\ 4/9\ text(of the cone volume.)`

Calculus, 2ADV C3 2006 HSC 9c

A cone is inscribed in a sphere of radius `a`, centred at `O`. The height of the cone is `x` and the radius of the base is `r`, as shown in the diagram.

Show that the volume, `V`, of the cone is given by

`V = 1/3 pi(2ax^2 - x^3)`. (2 marks)

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Find the value of `x` for which the volume of the cone is a maximum. You must give reasons why your value of `x` gives the maximum volume. (3 marks)

--- 8 WORK AREA LINES (style=lined) ---

Show Answers Only

a. `text(Proof)\ \ text{(See Worked Solutions)}`

b. `x = 4/3 a`

Show Worked Solution

a. `text(Show)\ V = 1/3 pi (2ax^2 – x^3)`

`V = 1/3 pi r^2 h`

`text(Using Pythagoras)`

`(x – a)^2 + r^2`	`= a^2`
`r^2`	`= a^2 – (x – a)^2`
	`= a^2 – x^2 + 2ax – a^2`
	`= 2ax – x^2`

`:. V`	`= 1/3 xx pi xx (2ax – x^2) xx x`
	`= 1/3 pi (2ax^2 – x^3)\ …\ text(as required)`

b. `(dV)/(dx) = 1/3 pi (4ax – 3x^2)`

`(d^2V)/(dx^2) = 1/3 pi (4a – 6x)`

`text(Max or min when)\ (dV)/(dx) = 0`

`1/3 pi (4ax – 3x^2)`	`= 0`
`4ax – 3x^2`	`= 0`
`x(4a – 3x)`	`= 0`

`3x`	`= 4a,`	` \ \ \ \ x ≠ 0`
`x`	` =4/3 a`

`text(When)\ \ x = 4/3 a`

`(d^2V)/(dx^2)`	`= 1/3 pi (4a – 6 xx 4/3 a)`
	`= 1/3 pi (-4a) < 0`
`=>\ text(MAX)`

`:.\ text(Cone volume is a maximum when)\ \ x = 4/3 a.`

Calculus, 2ADV C3 2005 HSC 8a

A cylinder of radius `x` and height `2h` is to be inscribed in a sphere of radius `R` centred at `O` as shown.

Show that the volume of the cylinder is given by

`V = 2pih(R^2 − h^2).` (1 mark)

--- 2 WORK AREA LINES (style=lined) ---
Hence, or otherwise, show that the cylinder has a maximum volume when `h = R/sqrt3.` (3 marks)

--- 6 WORK AREA LINES (style=lined) ---

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a. `text{Proof (See Worked Solutions).}`

b. `text{Proof (See Worked Solutions).}`

Show Worked Solution

a.	`V`	`= pir^2h\ \ \ \ text{(general form)}`
		`= pix^2 xx 2h`

`text(Using Pythagoras)`

`R^2`	`= h^2 + x^2`
`x^2`	`= R^2 − h^2`
`:.V`	`= 2pih(R^2 − h^2)\ \ …text(as required.)`

b.	`V`	`= 2pih(R^2 − h^2)`
		`= 2piR^2h − 2pih^3`
	`(dV)/(dh)`	`= 2piR^2 − 3 xx 2pih^2`
		`= 2piR^2 − 6pih^2`
	`(d^2V)/(dh^2)`	`= −12pih`

`text(Max or min when)\ (dV)/(dh) = 0`

`2piR^2 − 6pih^2`	`= 0`
`6pih^2`	`= 2piR^2`
`h^2`	`= R^2/3`
`h`	`= R/sqrt3,\ \ h > 0`

`text(When)\ h = R/sqrt3`

`(d^2V)/(dh^2) = −12pi xx R/sqrt3 < 0`

`:.\ text(Volume is a maximum when)\ h = R/sqrt3\ text(units.)`

Calculus, 2ADV C3 2014 HSC 16c

The diagram shows a window consisting of two sections. The top section is a semicircle of diameter `x` m. The bottom section is a rectangle of width `x` m and height `y` m.

The entire frame of the window, including the piece that separates the two sections, is made using 10 m of thin metal.

The semicircular section is made of coloured glass and the rectangular section is made of clear glass.

Under test conditions the amount of light coming through one square metre of the coloured glass is 1 unit and the amount of light coming through one square metre of the clear glass is 3 units.

The total amount of light coming through the window under test conditions is `L` units.

Show that `y = 5 - x(1 + pi/4)`. (2 marks)

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Show that `L = 15x - x^2 (3 + (5pi)/8)`. (2 marks)

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Find the values of `x` and `y` that maximise the amount of light coming through the window under test conditions. (3 marks)

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Show Answers Only

a. `text(Proof)\ \ text{(See Worked Solutions)}`

b. `text(Proof)\ \ text{(See Worked Solutions)}`

c. `x = 1.511\ text(m and)\ \ y = 2.302\ text(m)`

Show Worked Solution

`text(Frame is 10m)`

`10`	`= 2x + 2y + 1/2 pi x`
`2y`	`= 10\ – 2x\ – 1/2 pi x`
`:.y`	`= 5\ – x\ – pi/4 x`
	`= 5\ – x (1 + pi/4)\ \ \ text(… as required.)`

♦ Mean mark 35%

b.	`text(Area)\ text{(clear)}`	`= x xx y`
	`text(Area)\ text{(colour)}`	`= 1/2 xx pi r^2`
		`= 1/2 xx pi (x/2)^2`
		`= (pi x^2)/8`

`:.\ L`	`= 3xy + ((pix^2)/8 xx 1)`
	`= 3x [5\ – x (1 + pi/4)] + (pix^2)/8\ \ \ text{(see part (i))}`
	`= 15x\ – 3x^2\ – (3x^2pi)/4 + (x^2 pi)/8`
	`= 15x\ – 3x^2\ – (5x^2 pi)/8`
	`= 15x\ – x^2 (3 + (5pi)/8)\ \ \ text(… as required)`

♦ Mean mark 38%
COMMENT: A sanity check for your answer could be to compare your answers to the perimeter restriction of 10m.

c.	`L`	`= 15x\ – x^2 (3 + (5pi)/8)`
	`(dL)/(dx)`	`= 15\ – 2x (3 + (5pi)/8)`
	`(d^2L)/(dx^2)`	`= -2 (3 + (5pi)/8)`

`text(Max or min when)\ (dL)/(dx) = 0`

`15\ – 2x (3x + (5pi)/8)=`	`0`
`2x (3 + (5pi)/8)=`	`15`
`x=`	`15/(2 (3 + (5pi)/8)`
`=`	`1.51103…`
`=`	`1.511\ \ \ text{(3 d.p.)}`

`text(S)text(ince)\ (d^2L)/(dx^2) < 0\ \ \ => text(MAX)`

`text(When)\ \ x`	`= 1.511`
`y`	`= 5\ – 1.511 (1 + pi/4)`
	`= 2.3022…`
	`= 2.302\ text{(3 d.p.)}`

`:.\ text(MAX light when)\ x = 1.511\ text(m)`

`text(and)\ y = 2.302\ text(m.)`

Calculus, 2ADV C3 2008 HSC 10b

The diagram shows two parallel brick walls `KJ` and `MN` joined by a fence from `J` to `M`. The wall `KJ` is `s` metres long and `/_KJM=alpha`. The fence `JM` is `l` metres long.

A new fence is to be built from `K` to a point `P` somewhere on `MN`. The new fence `KP` will cross the original fence `JM` at `O`.

Let `OJ=x` metres, where `0<x<l`.

Show that the total area, `A` square metres, enclosed by `DeltaOKJ` and `DeltaOMP` is given by

`A=s(x-l+l^2/(2x))sin alpha`. (3 marks)

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Find the value of `x` that makes `A` as small as possible. Justify the fact that this value of `x` gives the minimum value for `A`. (3 marks)

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Hence, find the length of `MP` when `A` is as small as possible. (1 mark)

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Show Answers Only

a. `text{Proof (See Worked Solutions)}`

b. `l/sqrt2`

c. `(sqrt2-1)s\ \ text(metres)`

Show Worked Solution

`A=text(Area)\ Delta OJK+text(Area)\ Delta OMP`

♦♦♦ Low mean marks highlighted (although exact data not available before 2009).

`text(Using sine rule)`

`text(Area)\ Delta OJK=1/2\ x s sin alpha`

`text(Area)\ DeltaOMP =>text(Need to find)\ \ MP`

`/_OKJ`	`=/_MPO\ \ text{(alternate angles,}\ MP\ text(\|\|)\ KJtext{)}`
`/_PMO`	`=/_OJK=alpha\ \ text{(alternate angles,}\ MP\ text(\|\|)\ KJtext{)}`

`:.\ DeltaOJK\ text(|||)\ Delta OMP\ \ text{(equiangular)}`

`=>x/s`	`=(l-x)/(MP)\ `	` text{(corresponding sides of similar triangles)}`
`MP`	`=(l-x)/x *s`

`text(Area)\ Delta\ OMP`	`=1/2 (l-x)* MP * sin alpha`
	`=1/2 (l-x)((l-x))/x s sin alpha`

`:. A`	`=1/2 xs sin alpha+1/2 (l-x)((l-x))/x* s sin alpha`
	`=s sin alpha(1/2 x+1/2 (l-x)*((l-x))/x)`
	`=s sin alpha(1/2 x+(l-x)^2/(2x))`
	`=s sin alpha(1/2 x+l^2/(2x)-l+1/2 x)`
	`=s(x-l+l^2/(2x))sin alpha\ \ \ \ text(… as required)`

b. `text(Find)\ x\ text(such that)\ A\ text(is a minimum)`

MARKER’S COMMENT: Students who could not complete part (i) are reminded that they can still proceed to part (ii) and attempt to differentiate the result given.
Note that `l` and `alpha` are constants when differentiating.

`A`	`=s(x-l+l^2/(2x))sin alpha`
`(dA)/(dx)`	`=s(1-l^2/(2x^2))sin alpha`

`text(MAX/MIN when)\ (dA)/(dx)=0`

`s(1-l^2/(2x^2))sin alpha`	`=0`
`l^2/(2x^2)`	`=1`
`2x^2`	`=l^2`
`x^2`	`=l^2/2`
`x`	`=l/sqrt2,\ \ \ x>0`

`(d^2A)/(dx^2)=s((l^2)/(2x^3))sin alpha`

`text(S)text(ince)\ \ 0<alpha<90°\ \ =>\ sin alpha>0,\ \ l>0\ \ text(and)\ \ x>0`

`(d^2A)/(dx^2)>0\ \ \ =>text(MIN at)\ \ x=l/sqrt2`

c. `text(S)text(ince)\ \ MP=((l-x))/x s\ \ text(and MIN when)\ \ x=l/sqrt2`

`MP`	`=((l-l/sqrt2)/(l/sqrt2))s xx sqrt2/sqrt2`
	`=((sqrt2 l-l))/l s`
	`=(sqrt2-1)s\ \ text(metres)`

`:.\ MP=(sqrt2-1)s\ \ text(metres when)\ A\ text(is a MIN.)`

Calculus, 2ADV C3 2009 HSC 9b

An oil rig, `S`, is 3 km offshore. A power station, `P`, is on the shore. A cable is to be laid from `P` to `S`. It costs $1000 per kilometre to lay the cable along the shore and $2600 per kilometre to lay the cable underwater from the shore to `S`.

The point `R` is the point on the shore closest to `S`, and the distance `PR` is 5 km.

The point `Q` is on the shore, at a distance of `x` km from `R`, as shown in the diagram.

Find the total cost of laying the cable in a straight line from `P` to `R` and then in a straight line from `R` to `S`. (1 mark)

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Find the cost of laying the cable in a straight line from `P` to `S`. (1 mark)

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Let `$C` be the total cost of laying the cable in a straight line from `P` to `Q`, and then in a straight line from `Q` to `S`.

Show that `C=1000(5-x+2.6sqrt(x^2+9))`. (2 marks)

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Find the minimum cost of laying the cable. (4 marks)

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New technology means that the cost of laying the cable underwater can be reduced to $1100 per kilometre.

Determine the path for laying the cable in order to minimise the cost in this case. (2 marks)

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a. `$12\ 800`

b. `$15\ 160`

c. `text{Proof (See Worked Solutions)}`

d. `$12\ 200`

e. `P\ text(to)\ S\ text(in a straight line.)`

Show Worked Solution

♦♦ Although specific data is unavailable for question parts, mean marks were 35% for Q9 in total.

a. `text(C)text(ost)`	`=(PRxx1000)+(SRxx2600)`
	`=(5xx1000)+(3xx2600)`
	`=12\ 800`

`:. text(C)text(ost is)\ $12\ 800`

b. `text(C)text(ost)=PSxx2600`

`text(Using Pythagoras:)`

`PS^2`	`=PR^2+SR^2`
	`=5^2+3^2`
	`=34`
`PS`	`=sqrt34`

`:.\ text(C)text(ost)`	`=sqrt34xx2600`
	`=15\ 160.474…`
	`=$15\ 160\ \ text{(nearest dollar)}`

c. `text(Show)\ \ C=1000(5-x+2.6sqrt(x^2+9))`

`text(C)text(ost)=(PQxx1000)+(QSxx2600)`

`PQ`	`=5-x`
`QS^2`	`=QR^2+SR^2`
	`=x^2+3^2`
`QS`	`=sqrt(x^2+9)`

`:.C`	`=(5-x)1000+sqrt(x^2+9)\ (2600)`
	`=1000(5-x+2.6sqrt(x^2+9))\ \ text(… as required)`

d. `text(Find the MIN cost of laying the cable)`

`C`	`=1000(5-x+2.6sqrt(x^2+9))`
`(dC)/(dx)`	`=1000(–1+2.6xx1/2xx2x(x^2+9)^(–1/2))`
	`=1000(–1+(2.6x)/(sqrt(x^2+9)))`

`text(MAX/MIN when)\ (dC)/(dx)=0`

IMPORTANT: Tougher derivative questions often require students to deal with multiple algebraic constants. See Worked Solutions in part (iv).

`1000(–1+(2.6x)/(sqrt(x^2+9)))=0`

`(2.6x)/sqrt(x^2+9)`	`=1`
`2.6x`	`=sqrt(x^2+9)`
`(2.6)^2x^2`	`=x^2+9`
`x^2(2.6^2-1)`	`=9`
`x^2`	`=9/5.76`
	`=1.5625`
`x`	`=1.25\ \ \ \ (x>0)`

MARKER’S COMMENT: Check the nature of the critical points in these type of questions. If using the first derivative test, make sure some actual values are substituted in.

`text(If)\ \ x=1,\ \ (dC)/(dx)<0`

`text(If)\ \ x=2,\ \ (dC)/(dx)>0`

`:.\ text(MIN when)\ \ x=1.25`

`C`	`=1000(5-1.25+2.6sqrt(1.25^2+9))`
	`=1000(122)`
	`=12\ 200`

`:.\ text(MIN cost is)\ $12\ 200\ text(when)\ x=1.25`

e. `text(Underwater cable now costs $1100 per km)`

`=>\ C`	`=1000(5-x)+1100sqrt(x^2+9)`
	`=1000(5-x+1.1sqrt(x^2+9))`
`(dC)/(dx)`	`=1000(–1+1.1xx1/2xx2x(x^2+9)^(-1/2))`
	`=1000(–1+(1.1x)/sqrt(x^2+9))`

`text(MAX/MIN when)\ (dC)/(dx)=0`

`1000(–1+(1.1x)/sqrt(x^2+9))`	`=0`
`(1.1x)/sqrt(x^2+9)`	`=1`
`1.1x`	`=sqrt(x^2+9)`
`1.1^2x^2`	`=x^2+9`
`x^2(1.1^2-1)`	`=9`
`x^2`	`=9/0.21`
`x`	`~~6.5\ text{km (to 1 d.p.)}`
	`=>\ text(no solution since)\ x<=5`

`text(If we lay cable)\ PR\ text(then)\ RS`

MARKER’S COMMENT: Many students failed to interpret a correct calculation of `x>5` as providing no solution.

`=>\ text(C)text(ost)=5xx1100+3xx1000=8500`

`text(If we lay cable directly underwater via)\ PS`

`=>\ text(C)text(ost)=sqrt34xx1100=6414.047…`

`:.\ text{MIN cost is $6414 by cabling directly from}\ P\ text(to)\ S`.

Calculus, 2ADV C3 2010 HSC 5a

A rainwater tank is to be designed in the shape of a cylinder with radius `r` metres and height `h` metres.

The volume of the tank is to be 10 cubic metres. Let `A` be the surface area of the tank, including its top and base, in square metres.

Given that `A=2pir^2+2pi r h`, show that `A=2 pi r^2+20/r`. (2 marks)

--- 4 WORK AREA LINES (style=lined) ---
Show that `A` has a minimum value and find the value of `r` for which the minimum occurs. (3 marks)

--- 6 WORK AREA LINES (style=lined) ---

Show Answers Only

a. `text{Proof (See Worked Solutions)}`

b. `r~~1.17\ text(metres)`

Show Worked Solution

a. `text(Show)\ A=2 pi r^2+20/r`

MARKER’S COMMENT: Students MUST know the volume formula for a cylinder. Those that did and stated `10=pi r^2 h` most often completed this question efficiently.

`text(S)text(ince)\ V=pi r^2h=10\ \ \ \ =>\ h=10/(pi r^2)`

`text(Substituting into)\ A`

`A`	`= pi r^2+2 pi r (10/(pi r^2))`
	`=2 pi r^2+20/r\ \ \ text(… as required)`

♦ Mean mark 44%
MARKER’S COMMENT: The “table method” or 1st derivative test for proving a minimum (i.e. showing how `(dA)/(dr)` changes sign) was also quite successful.

b.	`A`	`=2 pi r^2+20/r`
	`(dA)/(dr)`	`=4 pi r-20/r^2`
	`(d^2A)/(dr^2)`	`=4 pi+40/r^3>0\ \ \ \ \ (r>0)`

`=>\ text(MIN occurs when)\ \ (dA)/(dr)=0`

`4 pi r-20/r^2`	`=0`
`4 pi r^3-20`	`=0`
`4 pi r^3`	`=20`
`r`	`=root3 (5/pi)`
	`=1.16754…`
	`=1.17\ text{metres (2 d.p.)}`

Calculus, 2ADV C3 2011 HSC 10b

A farmer is fencing a paddock using `P` metres of fencing. The paddock is to be in the shape of a sector of a circle with radius `r` and sector angle `theta` in radians, as shown in the diagram.

Show that the length of fencing required to fence the perimeter of the paddock is

`P=r(theta+2)`. (1 mark)

--- 2 WORK AREA LINES (style=lined) ---
Show that the area of the sector is `A=1/2 Pr-r^2`. (1 mark)

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Find the radius of the sector, in terms of `P`, that will maximise the area of the paddock. (2 marks)

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Find the angle `theta` that gives the maximum area of the paddock. (1 mark)

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Explain why it is only possible to construct a paddock in the shape of a sector if

`P/(2(pi+1)) <\ r\ <P/2` (2 marks)

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Show Answers Only

a. `text{Proof (See Worked Solutions)}`

b. `text{Proof (See Worked Solutions)}`

c. `r=P/4`

d. `2\ text(radians)`

e. `text{Proof (See Worked Solutions)}`

Show Worked Solution

a. `text(Need to show)\ \ P=r(theta+2)`

`P`	`=(2xxr)+theta/(2pi)xx2pir`
	`=2r+ r theta`
	`=r(theta+2)\ \ text(… as required)`

b. `text(Need to show)\ \ A=1/2 Pr-r^2`.

`text(S)text(ince)\ \ P=r(theta+2)\ \ \ =>\ theta=(P-2r)/r`

♦ Mean mark 41%.
TIP: Area of a sector `=pi r^2 xx` the percentage of the circle that the sector angle accounts for, i.e. `theta/(2 pi)` radians. `:. A=pi r^2“ xx theta/(2 pi)=1/2 r^2 theta`.

`:. A`	`=1/2 r^2 theta`
	`=1/2 r^2*(P-2r)/r`
	`=1/2(Pr-2r^2)`
	`=1/2 Pr-r^2\ \ text(… as required)`

c. `A=1/2 Pr-r^2`

♦♦ Mean mark 29%
MARKER’S COMMENT: The second derivative test proved much more successful and easily proven in this part. Make sure you are comfortable choosing between it and the 1st derivative test depending on the required calcs.

`(dA)/(dr)=1/2 P-2r`

`text(MAX or MIN when)\ \ (dA)/(dr)=0`

`1/2 P-2r`	`=0`
`2r`	`=1/2 P`
`r`	`=P/4`

`(d^2A)/(dr^2)=-2\ \ \ \ =>text(MAX)`

`:.\ text(Area is a MAX when)\ r=P/4\ text(units)`

d. `text(Need to find)\ theta\ text(when area is a MAX)\ =>\ r=P/4`

♦♦♦ Mean mark 20%

`P`	`=r(theta+2)`
	`=P/4(theta+2)`
`4P`	`=P(theta+2)`
`theta+2`	`=4`
`theta`	`=2\ text(radians)`

v. `text(For a sector to exist)\ \ 0<\ theta\ <2pi, \ \ text(and)\ \ theta=(P-2r)/r`

♦♦♦ Mean mark 4%. A BEAST!
MARKER’S COMMENT: When asked to ‘explain’, students should support their answer with a mathematical argument.

`=>(P-2r)/r`	`>0`
`P-2r`	`>0`
`r`	`<P/2`

`=>(P-2r)/r`	`<2pi`
`P-2r`	`<2r pi`
`2r pi+2r`	`>P`
`2r(pi+1)`	`>P`
`r`	`>P/(2(pi+1))`

`:.P/(2(pi+1)) <\ r\ <P/2\ \ text(… as required)`

Calculus, 2ADV C3 2012 HSC 16b

The diagram shows a point `T` on the unit circle `x^2+y^2=1` at an angle `theta` from the positive `x`-axis, where `0<theta<pi/2`.

The tangent to the circle at `T` is perpendicular to `OT`, and intersects the `x`-axis at `P`, and the line `y=1` intersects the `y`-axis at `B`.

Show that the equation of the line `PT` is `xcostheta+ysin theta=1`. (2 marks)

--- 5 WORK AREA LINES (style=lined) ---
Find the length of `BQ` in terms of `theta`. (1 mark)

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Show that the area, `A`, of the trapezium `OPQB` is given by

`A=(2-sintheta)/(2costheta)` (2 marks)

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Find the angle `theta` that gives the minimum area of the trapezium. (3 marks)

--- 6 WORK AREA LINES (style=lined) ---

Show Answers Only

a. `text{Proof (See Worked Solutions)}`

b. `(1-sin theta)/cos theta`

c. `text{Proof (See Worked Solutions)}`

d. `theta=pi/6\ \ text(radians)`

Show Worked Solution

`text(Find)\ T:`

♦♦ Mean mark 20%

`text(S)text(ince)\ \ cos theta=x/1\ \ \ text(and)\ \ \ sin theta=y/1`

`:. T\ (cos theta, sin theta)`

`text(Gradient of)\ OT=sin theta/cos theta`

`:.\ text(Gradient)\ PT=-cos theta/sin theta\ \ text{(} _|_ text{lines)}`

`text(Equation of)\ PT\ text(where)`

`m=-cos theta/sin theta,\ \ text(and through)\ \ (cos theta, sin theta)`

`text(Using)\ \ y-y_1`	`=m(x-x_1)`
`y-sin theta`	`=-cos theta/sin theta(x-cos theta)`
`y sin theta-sin^2 theta`	`=-x cos theta+cos^2 theta`
`x cos theta+y sin theta`	`=sin^2 theta+cos^2 theta`
`x cos theta+y sin theta`	`=1\ \ \ \ \ text(… as required)`

b. `text(Find)\ Q:`

`Q\ => text(intersection of)\ xcos theta+y sin theta=1\ \ text(and)\ \ y=1`

`x cos theta+sin theta`	`=1`
`x cos theta`	`=1-sin theta`
`x`	`=(1-sin theta)/cos theta`

`:.\ text(Length of)\ BQ\ text(is)\ \ (1-sin theta)/cos theta\ text(units)`

c. `text(Show Area)\ OPQB=(2-sin theta)/(2cos theta)`

`A=1/2h(a+b)\ \ text(where)\ \ h=OB=1\ \ a=OP\ \ text(and)`

`b=BQ=(1-sin theta)/cos theta`

`text(Find length)\ OP:`

`P => xcos theta+ysin theta=1 \ text(cuts)\ \ x text(-axis)`

♦♦ Mean mark 24%

`xcos theta`	`=1`
`x`	`=1/cos theta`
`=>text(Length)\ OP`	`=1/cos theta`

`text(Area)\ OPQB`	`=1/2xx1(1/cos theta+(1-sin theta)/cos theta)`
	`=1/2((2-sin theta)/cos theta)`
	`=(2-sin theta)/(2cos theta)\ \ text(u²)\ \ \ text(… as required)`

d. `text(Find)\ theta\ text(such that Area)\ OPQB\ text(is a MIN)`

`A`	`=(2-sin theta)/(2cos theta)`
`(dA)/(d theta)`	`=(2cos theta(-cos theta)-(2- sin theta)(-2 sin theta))/(4cos^2 theta)`
	`=(4 sin theta-2sin^2 theta-2 cos^2 theta)/(4 cos^2 theta)`
	`=(4sin theta-2(sin^2 theta+cos^2 theta))/(4 cos^2 theta)`
	`=(4sin theta-2)/(4cos^2 theta)`
	`=(2sin theta-1)/(2 cos^2 theta)`

Mean mark 19%
IMPORTANT: Look for any opportunity to use the identity `sin^2 theta“+cos^2 theta=1` → often a key to simplifying difficult trig equations.

`text(MAX or MIN when)\ (dA)/(d theta)=0`

`=>2sin theta-1`	`=0`
`sin theta`	`=1/2`
`theta`	`=pi/6\ \ \ \ \0<theta<pi/2`

`text(Test for MAX/MIN)`

IMPORTANT: Is the 1st or 2nd derivative test easier here? Examiners have often made one significantly easier than the other.

`text(If)\ theta=pi/12\ \ (dA)/(d theta)<0`

`text(If)\ theta=pi/3\ \ (dA)/(d theta)>0\ \ =>text(MIN)`

`:.\text(Area)\ OPQB\ text(is a MIN when)\ theta=pi/6`.

Calculus, 2ADV C3 2013 HSC 14b

Two straight roads meet at `R` at an angle of 60°. At time `t=0` car `A` leaves `R` on one road, and car `B` is 100km from `R` on the other road. Car `A` travels away from `R` at a speed of 80 km/h, and car `B` travels towards `R` at a speed of 50 km/h.

The distance between the cars at time `t` hours is `r` km.

Show that `r^2=12\ 900t^2-18\ 000t+10\ 000`. (2 marks)

--- 5 WORK AREA LINES (style=lined) ---
Find the minimum distance between the cars. (3 marks)

--- 7 WORK AREA LINES (style=lined) ---

Show Answers Only

a. `text{Proof (See Worked Solutions)}`

b. `61\ text(km)`

Show Worked Solution

a. `text(Need to show)\ r^2=12\ 900t^2-18\ 000t+10\ 000`

♦♦ Mean mark 26%

`RB=100-50t`

`RA=80t`

`text(Using the cosine rule)`

`r^2`	`=(RB)^2+(RA)^2-2(RB)(RA)cos/_R`
	`=(100-50t)^2+(80t)^2-2(100-50t)(80t)cos60`
	`=10\ 000-10\ 000t+2500t^2+6400t^2-8000t+4000t^2`
	`=12\ 900t^2-18\ 000t+10\ 000\ \ \ \ text(… as required)`

b. `text(Max/min when)\ (dr^2)/(dt)=0`

♦♦ Mean mark 27%
ALGEBRA TIP: Finding the derivative of `r^2` (rather than making `r` the subject), makes calculations much easier. ENSURE you apply the test to confirm a minimum.

`(dr^2)/(dt)=25\ 800t-18\ 000=0`

`t=(18\ 000)/(25\ 800)=30/43`

`text(When)\ t=30/43,\ (d^2(r^2))/(dt^2)=25\ 800>0\ \ =>text(MIN)`

`:.\ text(Minimum distance when)\ t=30/43\ text(hr)`

`text(Find)\ r\ text(when)\ t=30/43`

`r^2`	`=12\ 900(30/43)^2-18\ 000(30/43)+10\ 000`
	`=3720.9302…`
`:.\ r`	`=sqrt3702.9302…`
	`~~60.99942…`
	`~~61\ text{km (nearest km)}`

`:.\ text(MIN distance between the cars is 61 km.)`

\(d\)	\(=\sqrt{(x-2)^2+(x+c)^2}\)
	\(=\sqrt{x^2-4x+4+x^2+2cx+c^2}\)
	\(=\sqrt{2x^2+(2c-4)x+4+c^2}\)

ii.	\(\dfrac{dA}{dx}\)	\(=2\sqrt{(100-x^2}-\dfrac{2x^2}{\sqrt{100-x^2}}\)
		\(=\dfrac{2(100-x^2)-2x^2}{\sqrt{100-x^2}}\)
		\(=\dfrac{4(50-x^2)}{\sqrt{100-x^2}}\)

\(\dfrac{4(50-x^2)}{\sqrt{100-x^2}}\)	\(=0\quad(x \neq 10)\)
\(50-x^2\)	\(=0\)
(\(\sqrt{50}-x)(\sqrt{50}+x)\)	\(=0\)
\(\therefore\ x\)	\(=\sqrt{50}=5\sqrt{2}\quad (0<x<10)\)

\(x\)	\(7\)	\(\sqrt{50}\)	\(7.2\)
\(A^{\prime}\)	\(0.56\)	\(0\)	\(-1.06\)
\(\text{Gradient}\)	\(+\)	\(0\)	\(-\)

\(A\)	\(=\dfrac{\theta}{2\pi} \times \pi (1+x)^2-\dfrac{\theta}{2\pi} \times \pi \times 1^2\)
\(A\)	\(=\dfrac{\theta}{2} (1+x)^2-\dfrac{\theta}{2}\)
\(A\)	\(=\dfrac{\theta}{2}(1+2x+x^2-1)\)
\(A\)	\(=\dfrac{\theta}{2}x(x+2)\)
\(\theta\)	\(=\dfrac{2A}{x(x+2)}\)

\(\text{Total Area}\)	\(=\dfrac{\sqrt{3} x^2}{4}+\dfrac{(100-3 x)^2}{4 \pi}\)
	\(=\dfrac{1}{4}\left(\sqrt{3} x^2+\dfrac{(100-3 x)^2}{\pi}\right)\)

\(A(x)\)	\(=\dfrac{1}{4}\left(\sqrt{3} x^2+\dfrac{10\,000}{\pi}-\dfrac{600 x}{\pi}+\dfrac{9 x^2}{\pi}\right)\)
	\(=\dfrac{1}{4}\left(\sqrt{3}+\dfrac{9}{\pi}\right) x^2-\dfrac{150}{\pi} x+\dfrac{2500}{\pi}\)

\(\text{Perimeter}\ QRST\)	\(=\dfrac{\theta}{2\pi} \times 2\pi(x+1)+\dfrac{\theta}{2\pi} \times 2\pi (1)+2x \)
	\(=\theta(x+1)+\theta+2x\)
	\(=\theta(x+2)+2x\)
	\(=\dfrac{2A}{x(x+2)}(x+2)+2x\)
	\(=2x+\dfrac{2A}{x}\)

\(2-\dfrac{2A}{x^2}\)	\(=0\)
\(2x^2\)	\(=2A\)
\(x^2\)	\(=A\)