# Rate of change of area of circle in respect to radius

1. Feb 25, 2014

### grace77

What is the Rate of change of area of circle in respect to radius when radius is 3in
I know that that dA/dr is equal to the circumference of the circle
But where does that come from?
Also the formula for the circumference of the circle is 2(pi)r
But the answer is 6 (pi)in^2/in.
I understand the 6 but where are the units coming from if the circumference of the circle will only give you units for r ?

Hope someone can shed some light on this for me! Thanks

2. Feb 25, 2014

### PhysicoRaj

It is rate of change of AREA wrt RADIUS. What are the units of these two?

3. Feb 25, 2014

### grace77

Ok so you basically use the formula for the circumference of the circle but use the units for area/radius?

4. Feb 25, 2014

### PhysicoRaj

Both are one and the same. But since we are representing the rate of change, it is better to use units like that.

5. Feb 25, 2014

### grace77

Thank you.

6. Feb 25, 2014

### Integral

Staff Emeritus
Homework questions need to be posted in the homework forums.