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??
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: does not refer to the annulus.