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*r*when

*r*=2." My first question is does the constant of proportionality refer to [tex]\frac{dV}{dA}[/tex]? Secondly, am I being asked to find the rate of change of the radius with respect to time [tex]\frac{dr}{dt}[/tex] or another rate?

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quasar987

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No, if you are told that A is proportional to B, it means there is a constant k such that A = kB. Here, it means that dV/dt = kA, for a certain constant k. Next, you are told that k = 3. But this is nonsense in my opinion because the raindrop is

Yes, the rate of change of radius wrt time is dr/dt.

- #3

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[tex]\frac{dV}{dt}=\frac{dV}{dr}\frac{dr}{dt}[/tex]

Since [tex]\frac{dV}{dt}=-3A[/tex]

Then [tex]-3(4{\pi}r^2)=\frac{dV}{dr}\frac{dr}{dt}[/tex]

But now it seems if i differentiate the volume with respect to the radius ill get the equation for surface area and it will cancel out and leave

[tex]\frac{dr}{dt}=-3[/tex]

And that cant be right, where have i erred?

- #4

quasar987

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Why can't it be right? I'd say it can't be *wrong*.

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