can someone help me solve this. Dont know how to start
The radius of a circular puddle is growing at a rate of 20 cm/s.
(a) How fast is its area growing at the instant when the radius is 40 cm?
(b) How fast is the area growing at the instant when it equals 1 cm2?

Question

can someone help me solve this. Dont know how to start
The radius of a circular puddle is growing at a rate of 20 cm/s.
(a) How fast is its area growing at the instant when the radius is 40 cm?
(b) How fast is the area growing at the instant when it equals 1 cm2?

anonymous · Answer

B

wikiemol · Answer

well the radius is 20t if t is time (assuming that the puddle has a radius of 0 at time 0), so its area at any given time is π400t^2. If you take the derivative of that you get π800t as the change in area.  so to find what time it is when the radius is 40 cm you solve this equation: 40 = 20t. t = 2 then. and if you plug t into the rate of change of the area π1600cm/s as the change. You can do the same thing for b.