Oil spilled from a ruptured tanker spreads in a circle whose area increases at a constant rate of 7.5 mi^2/hr. How rapidly is radius of the spill increasing when the area is 8 mi^2?

Question

I tried using A=(pi)r^2
and dA/dt=2(pi)r(dr/dt)

and plugging in 7.5=2(pi)8(dr/dt)

but the answer is incorrect. Please help!

Answer 1

pi r^2 = 8
so
r = 1.60 miles

dA/dt = 2 pi r dr/dt yes, correct
so
7.5 = 2 pi (1.6) dr/dt

dr/dt = .748 miles/hr

You used the area, 8 for the radius R

Answer 2

HOW DID YOU USED R=1.60 CM