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Calculus, EXT1 C1 2017 HSC 8 MC

A stone drops into a pond, creating a circular ripple. The radius of the ripple increases from 0 cm, at a constant rate of `5\ text(cm s)^(−1)`.

At what rate is the area enclosed within the ripple increasing when the radius is 15 cm?

A.     `25pi\ text(cm)^2\ text(s)^(−1)`

B.     `30pi\ text(cm)^2\ text(s)^(−1)`

C.     `150pi\ text(cm)^2\ text(s)^(−1)`

D.     `225pi\ text(cm)^2\ text(s)^(−1)`

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`C`

Show Worked Solution

`(dr)/(dt) = 5\ text(cm)^2\ text(s)^(−1)`

`A` `=pi r^2`
`(dA)/(dr)` `= 2pir`

 

`(dA)/(dt)` `= (dA)/(dr) · (dr)/(dt)`
  `= 2pi r · 5`
  `= 10pir`

 
`text(When)\ r = 15`

`(dA)/(dr)` `= 10pi · 15`
   `= 150pi\ text(cm)^2\ text(s)^(−1)`

 
`=>C`

Calculus, EXT1 C1 2013 HSC 13a

A spherical raindrop of radius `r` metres loses water through evaporation at a rate that depends on its surface area.  The rate of change of the volume `V` of the raindrop is given by

`(dV)/(dt) = -10^(-4) A`, 

where `t` is time in seconds and `A` is the surface area of the raindrop. The surface area and the volume of the raindrop are given by  `A = 4pir^2`  and  `V = 4/3 pi r^3`  respectively.

  1. Show that  `(dr)/(dt)`  is constant.   (1 mark)

  2. How long does it take for a raindrop of volume  `10^(–6)` m3  to completely evaporate?     (2 marks)

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  1. `text(Proof)\ \ text{(See Worked Solutions)}`
  2. `62\ text(seconds)`
Show Worked Solution
i.    `text(Show)\ \ (dr)/(dt)\ \ text(is a constant)`

 `(dV)/(dt) = (dV)/(dr) * (dr)/(dt)\ \ \ …\ text{(1)}`

`V` `= 4/3 pi r^3`
`:. (dV)/(dr)` `= 4 pi r^2`
`(dV)/(dt)` `= -10^(-4) A\ \ text{(given)}`

 
`text(Substituting into)\ text{(1)}`

`-10^(-4) A` `= 4 pi r^2 xx (dr)/(dt)`
  `= A xx (dr)/(dt)`
`:.\ (dr)/(dt)` `= -10^(-4)\ \ text(… as required)`

 

ii.    `V` `= 10^(-6)\ text(m³)`
  `4/3 pi r^3` `= 10^(-6)`
  `r^3` `= (3 xx 10^(-6))/(4pi)`
  `r` `= root(3)((3 xx 10^(-6))/(4pi))`

 

`text(S)text(ince the radius decreases at a constant rate,)`

♦♦ Mean mark 31%

`t=(root(3)((3 xx 10^(-6))/(4pi)))/(10^(-4))`

`\ \ =62.035 …`

`\ \ =62\ text(seconds)\ text{(nearest whole)}`

 

`:.\ text(It takes 62 seconds for the raindrop)`

`text(to evaporate.)`