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# Need help with this math question?

The radius of a circle increases at a rate of 0.5cms^-1. Find the rate of change in the area of the circle when the area is 25π cm^2

Answer given is 5π cm^2s^-1

I think to know the step to get the answer

### 1 Answer

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- billrussell42Lv 72 months agoFavourite answer
A = πr²

dA/dr = 2πr

dr/dt = 0.5 cm/s

dA/dt = dA/dr x dr/dt = (0.5)(2πr) = πr

when A = 25π, r = √(A/π) = √(25π/π) = 5

dA/dt = πr = 5π cm²/s

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A = πr², solve for r