word problems

[jon12]

A stone is tossed inot a calm pond and circular ripple it creates has a radius that is increasing at a rate of 4 centimeters per second. Find the rate of change of the area when the radius of the circle is 24 centimeters.

Need some help with this word problem

 

[galactus]

Implicitly differentiate the formula for the area of a circle.

That is, differentiate the area of a circle formula and attach a dr/dt to get dA/dt.

Then, enter in the givens and you have it.

You are given dr/dt and r. You need dA/dt.