1. Jan 4, 2009

### hunter55

1. The problem statement, all variables and given/known data
a snowball is rolling donw a hill, its radius is changing at a rate of 2 cm/min. what is the rate of change of the area, when the radius is 8 cm?

2. Relevant equations

da/dt= 2pir2dr/dt
dat/dt=2pi(8)(2)
32pi ???
is that right

3. The attempt at a solution

32pi

2. Jan 4, 2009

### Dick

What formula are you using for A? If you are thinking of the snowball as a disk and A=pi*r^2, then yes, dA/dt=pi*2*r*dr/dt. You might want to put units on your answer.

3. Jan 5, 2009

### HallsofIvy

Staff Emeritus
As Dick suggested, you are using the wrong formula (because a snowball is NOT a disk). The "area'' referred to here is the SURFACE area of a sphere.
Surface area of a sphere is $4\pi r^2$.