Why is the derivative of a circle's area its perimeter?

[t246335]

Homework Statement

Explain why the relationship of the derivative of a Circle's area is its perimeter makes sense.

Homework Equations

Area of a Cirle=pi*r^2

The Attempt at a Solution

da/dr=2pi*r
2r=diameter and the circumfrece=pi*D so da/dr=Circumfrence
How do i explain why this works?

 

[Dick]

No idea? Draw a circle of radius r and a circle of radius r+dr. Can't you see a geometric way to approximate the difference in the areas?