# Thread: Related rates problem

1. ## Related rates problem

Let be the area of a circle with radius . If , find when .
Thanks

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2. $\dfrac{dA}{dt} = \dfrac{dA}{dr} \times \dfrac{dr}{dt}$

You know dr/dt.

You know that $A = \pi r^2$

Find dA/dr and plug in r = 1 and plug that in the chain rule equation.

3. $A=\pi\;r^2$

$\dfrac{dA}{dt} = 2\pi r \; \dfrac{dr}{dt}$