# The area `A(r) = pir^2` of a circular oil spill changes with the radius. At what rate does the area change with respect to the radius when r = 10ft?

To find the instantaneous rate of change of a function at a point, we take the first derivative of the function and evaluate the derivative at that point.

`A(r)=pi r^2`

`(dA)/(dr)=2pir` so `A'(r)=2pir`