Calculus Problem: Please Help!!

Question

The area A = πr² of a circular water ripple changes with the radius. At what rate does the area change with respect to the radius when r = 4ft? 

Options:

1. 16π ft²/ft
2. 8π ft²/ft
3. 8 ft²/ft
4. 4π ft²/ft

Answer 1

Since A = πr², dA/dr = 2πr. Just plug in the number to get the answer.