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

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

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

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

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

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

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

Thank you.

Integral
Staff Emeritus