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# how do i do these question? c3 maths rates of change, HELP HELP Watch

1. sorry

okay heres the question:

2. Can you show what you've attempted, if anything?

To start off, you should write down the expression for the area of the circle. Differentiating that expression will give you the rate of change of the area with respect to the radius.
3. (Original post by Edgemaster)
Can you show what you've attempted, if anything?
i know you have to use the chain rule, but honestly i just don't understand how to

i think it's dr/dt = dt/dr*da/dr ??

im sorry idk ;(
4. (Original post by Edgemaster)
Can you show what you've attempted, if anything?

To start off, you should write down the expression for the area of the circle. Differentiating that expression will give you the rate of change of the area with respect to the radius.
??
5. Okay, so chain rule is . We know the rate of the increase in the radius (0.2cm/s), which is dr/dt. dA/dt is also known (20cm^2/s).

$20=\frac{dA}{dr}\cdot 0.2$

And you have an expression for dA/dr (differentiate A = πr^2). You can then solve for r.

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