# Math Help - ring question..

1. ## ring question..

basically what i need is to find the area of the ring
the with is dr and the radius is r
so dA=pi*(r+dr)^2 - pi*(r)^2
so why the dA is that simple expression??

http://www.freeimagehosting.net/imag...cce537487f.gif

2. Originally Posted by transgalactic
basically what i need is to find the area of the ring
the width is dr and the radius is r
so dA=pi*(r+dr)^2 - pi*(r)^2
so why the dA in that simple expression??
Your equation above is correct for the annulus.
However, in the image: $\sigma d A$ does not refer to the annulus.

3. its a part of anulus
whats the logic of calculating area in double integral

4. Originally Posted by aidan
Your equation above is correct for the annulus.
However, in the image: $\sigma d A$ does not refer to the annulus.
$2\pi{rdr}$ is an approximate value. The $(dr)^2$ is omitted.

5. this is the simple way
i want to know how to calculate that with double integral