Using polar coordinates, evaluate the integral which gives the area which lies in the first quadrant between the circles

and

i tried setting it up like this but could get the right answers

r^2 = 400

r = 20

r^2 = 20rcosΘr = 20cosΘ

Mr F says: This is just plain sloppy. r^2 does NOT equal 20cosΘ. r = 20cosΘ.
i dont know how to use math tags

but my integral for dΘ would be from 0 to 2pi

Mr F says: What makes you think that? Have you drawn a diagram that shows the required area?
and my integral for dr would be from 20cosΘ to 20i integrated the f(x,y) = 1, because that is volume, right?

Mr F says: Where is volume mentioned in the question? Nowhere that I can see. So why are you integrating wrt r as well? The question asks for an area ......Do you know the formula for the area between two polar curves?