Let \(f(x)=6 \sqrt{x+1}+5\).
Find the gradient of the tangent to \(y=f(x)\) at \(x=8\). (2 marks)
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Let \(f(x)=6 \sqrt{x+1}+5\).
Find the gradient of the tangent to \(y=f(x)\) at \(x=8\). (2 marks)
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\(\text{Gradient of tangent }=1\)
| \(f(x)\) | \(=6 \sqrt{x+1}+5\) |
| \(f^{\prime}(x)\) | \(=6 \times \dfrac{1}{2} \times(x+1)^{-\tfrac{1}{2}}=\dfrac{3}{\sqrt{x+1}}\) |
\(\text{At} \ \ x=8:\)
\(f^{\prime}(x)=\dfrac{3}{\sqrt{8+1}}=1\)
\(\therefore \ \text{Gradient of tangent }=1\)
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a. `y^{′} = 4x+5`
b. `y = 9x-2`
| a. | `f(x)` | `= 2x^2 + 5x` |
| `f^{′}(x)` | `= lim_(h->0) (f(x + h)-f(x))/h` | |
| `= lim_(h->0) ((2(x + h)^2 +5(x + h))-(2x^2+5x))/h` | ||
| `= lim_(h->0)(2x^2 + 4xh + 2h^2+5x+5h-2x^2-5x)/h` | ||
| `= lim_(h->0)(4xh + 2h^2+5h)/h` | ||
| `= lim_(h->0)(h(4x+5 +2h))/h` |
`:.\ y^{′} = 4x+5`
b. `text(When)\ \ x = 1, y = 7`
`y^{′} = 4+5 = 9`
| `y-7` | `= 9(x-1)` |
| `y` | `= 9x-2` |
At which point on the curve \(y=2x^{2}-11x+3\) can a tangent be drawn such that it is inclined at 45° when it crosses the positive \(x\)-axis?
\(D\)
| \(y\) | \(=2x^{2}-11x+3\) | |
| \(y^{′}\) | \(=4x-11\) |
\(\text{If a tangent crosses (positive) x-axis at 45}^{\circ},\)
\(m_{\text{tang}}=1 \ \ (\tan 45^{\circ}=1) \)
\(\text{Find}\ x\ \text{when}\ \ y^{′}=1: \)
| \(4x-11\) | \(=1\) | |
| \(4x\) | \(=12\) | |
| \(x\) | \(=3\) |
\(\text{Tangent at point}\ (3,-12) \)
\(\Rightarrow D\)
Find the equation of the tangent to the curve `y=(2x+1)^3` at the point `(0,1)`. (3 marks)
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`y=6x+1`
| `y` | `=(2x+1)^3` | |
| `dy/dx` | `=3xx2(2x+1)^2=6(2x+1)^2` |
`text{At}\ x=0\ \ =>\ \ dy/dx=6xx1^2=6`
`text{Find equation of line}\ m=6, text{through}\ (0,1):`
| `y-y_1` | `=m(x-x_1)` | |
| `y-1` | `=6(x-0)` | |
| `y` | `=6x+1` |
Let `h(x)=(f(x))/(g(x))`, where
`{:[f(1)=2, qquad f^{′}(1)=4],[g(1)=8, qquad g^{′}(1)=12]:}`
What is the gradient of the tangent to the graph of `y=h(x)` at `x=1` ?
`D`
`text{Using the quotient rule:}`
| `h^{′}(1)` | `=(g(1)\ f^{′}(1)-f(1)\ g^{′}(1))/[g(1)]^2` | |
| `=(8xx4-2xx12)/(8^2)` | ||
| `=(32-24)/64` | ||
| `=1/8` |
`=>D`
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a. `y = -3x, \ y = 3x-9`
b. `(3/2, -9/4)`
a. `y= x^2-3x= x(x-3)`
`text(Cuts)\ xtext(-axis at)\ \ x = 0\ \ text(or)\ \ x = 3`
`(dy)/(dx) = 2x-3`
`text(At)\ \ x = 0 \ => \ (dy)/(dx) = -3`
`T_1\ text(has)\ \ m = -3,\ text{through (0, 0):}`
| `y-0` | `= -3(x-0)` |
| `y` | `= -3x` |
`text(At)\ \ x = 3 \ => \ (dy)/(dx) = 3`
`T_2\ text(has)\ \ m = 3,\ text{through (3, 0):}`
| `y-0` | `= 3(x-3)` |
| `y` | `= 3x-9` |
b. `text(Intersection occurs when:)`
| `3x-9` | `= -3x` |
| `6x` | `= 9` |
| `x` | `= 3/2` |
`y = -3 xx 3/2 = -9/2`
`:.\ text(Intersection at)\ \ (3/2, -9/2)`
Find the equation of the tangent to the curve `y = x^2 + 4x - 7` at the point `(1, -2)`. (2 marks)
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`y = 6x – 8`
| `y` | `= x^2 + 4x – 7` |
| `(dy)/(dx)` | `= 2x + 4` |
`text(When)\ x = 1,\ \ (dy)/(dx) = 6`
`text(Equation of tangent through)\ (1, -2)`
| `y + 2` | `= 6 (x – 1)` |
| `y` | `= 6x – 8` |
Find the gradient of the tangent to the curve `y = x^4- 3x` at the point `(1, –2)`. (2 marks)
`text(Gradient = 1.`
| `y` | `= x^4\ – 3x` |
| `dy/dx` | `= 4x^3\ – 3` |
`text(At)\ x = 1`
`dy/dx = 4\ – 3 = 1`
`:.\ text(Gradient of tangent at)\ (1,–2) = 1.`
The parabola shown in the diagram is the graph `y = x^2`. The points `A (–1,1)` and `B (2, 4)` are on the parabola.
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There is a point `C` on the parabola such that the tangent at `C` is parallel to `AB`.
Show that the line `MC` is vertical. (2 marks)
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Show that the line `BT` is a tangent to the parabola. (2 marks)
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| i. | ![]() |
`y=x^2\ \ =>\ \ dy/dx=2x`
`text(At)\ \ A(-1,1)\ \ =>\ \ dy/dx = -2`
`text(T)text(angent has)\ \ m=text(–2),\ text(through)\ text{(–1,1):}`
| `y-y_1` | `= m(x-x_1)` |
| `y-1` | `= -2 (x + 1)` |
| `y-1` | `= -2x-2` |
| `2x + y + 1` | `= 0` |
`:.\ text(T)text(angent at)\ A\ text(is)\ \ 2x + y + 1 = 0`
ii. `Atext{(–1,1)}\ \ \ B(2,4)`
`M= ((-1+2)/2 , (1+4)/2)= (1/2, 5/2)`
`m_(AB)= (y_2-y_1)/(x_2-x_1)= (4-1)/(2 + 1)=1`
`text(When)\ \ dy/dx=1:`
`2x=1\ \ =>\ \ x=1/2\ \ =>\ \ C \ (1/2, 1/4)`
`=>M\ text(and)\ C\ text(both have)\ x text(-value)=1/2`
`:. MC\ text(is vertical … as required)`
iii. `T\ text(is point on tangent when) \ x=1/2`
`text(T)text(angent)\ \ \ 2x + y + 1 = 0`
`text(At)\ x = 1/2`
`2 xx (1/2) + y + 1=0\ \ =>\ \ y=–2`
`:.\ T (1/2, –2)`
`text (Given)\ B (2, 4):`
`m_(BT)= (4+2)/(2-1/2)=4`
`text(At)\ \ B(2,4),\ text(find gradient of tangent:)`
`dy/dx = 2x=2 xx2=4`
`m_text(tangent) = 4=m_(BT)`
`:.BT\ text(is a tangent)`
Find the equation of the tangent to the curve `y = (2x + 1)^4` at the point where `x = -1`. (3 marks)
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`8x + y + 7 = 0`
`y = (2x + 1)^4`
`text{Using the Chain Rule:}`
`dy/dx= 4 xx (2x + 1)^3 xx d/dx (2x + 1)= 8 (2x + 1)^3`
`text(At)\ \ x = -1:`
`dy/dx= 8 (2(-1) + 1)^38 (-1)^3= -8`
`text{Tangent has}\ m = -8\ text(through)\ (-1,1):`
| `y-y_1` | `= m (x-x_1)` |
| `y-1` | `= -8 (x + 1)` |
| `y-1` | `= -8x -8` |
| `8x + y + 7` | `= 0` |
`:.\ text(Equation of tangent is)\ 8x + y + 7 = 0`
Find the equation of the tangent to the curve `y = x^2` at the point where `x = 3`. (2 marks)
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`6x-y-9 = 0 `
`y=x^2\ \ =>\ \ dy/dx=2x`
`text{Tangent equation has}\ m = 6, \text{through}\ (3,9):`
`text(When) \ x = 3, y = 9\ \ =>\ \ dy/dx = 6`
| `y-y_1` | `= m (x-x_1)` |
| `y-9` | `= 6(x-3)` |
| `y-9` | `= 6x-18` |
| `6x-y-9` | `=0` |