# Homework Help: Rate of change

1. Feb 16, 2008

### Incog

1. The problem statement, all variables and given/known data

A liquid is being poured onto a level surface making a circular pattern on the surface. Find the rate of change of the area covered on the surface with respect to the radius when the radius is 20cm.

2. Relevant equations

Surface area = (Pi)r^2

3. The attempt at a solution

Well, what's there to do? If you find the surface area, it's 1256 but what do you do after that? Only one numerical value is given so there's not much to work with. And I don't even know how the answer's supposed to look like - is it gonna be in cm, cm^2...Can somebody get me started here...

2. Feb 16, 2008

### pooface

Whenever you see that RATE OF CHANGE is being asked of, then this should indicate that you should be using derivatives.

Now it is asking for rate of change of the Surface area. So find d(SA)/dt.

3. Feb 16, 2008

### ||spoon||

The question is asking you how quickly is the area of the puddle increasing, given that the puddle already has a radius of 20cm.