The rate a which the surface area, S, decreases is
The surface area is
S = 4πr²
where r = the radius at time t.
Therefore
When r = 3 cm, obtain
![\frac{dr}{dt}]_{r=3} = - \frac{1}{3} \, cm/s](https://tex.z-dn.net/?f=%20%5Cfrac%7Bdr%7D%7Bdt%7D%5D_%7Br%3D3%7D%20%3D%20-%20%5Cfrac%7B1%7D%7B3%7D%20%5C%2C%20cm%2Fs%20%20)
Answer: -1/3 cm/s (or -0.333 cm/s)
The surface area is
S = 4πr²
where r = the radius at time t.
Therefore
When r = 3 cm, obtain
Answer: -1/3 cm/s (or -0.333 cm/s)