The radius in meters of a circular oil slick increases at a rate of (12 - 0.3t), where t is measured in hours. Which of the following expressions would calculate the rate at which the area of the oil slick is increasing when the radius is 100 meters?

Answer 1

dr/dt = (12 - .3 t)

a = pi r^2
da/dt = 2 pi r dr/dt

da/dt = 2 pi r (12-.3 t)
when r = 100
da/dt = 200 pi (12 - .3 t)