ok ummmmmmmmmmmmmmmmmmmmm so we have a wedge covering an angle of dtheta the thickness of the wedge is roughly rdtheta
the wedge can be thought of as a stack of triangles where each triangle umm - is r long - is r dtheta wide [fixed mistake with editing] - has an area of 1/2 base * height = 1/2 r * r dtheta = 1/2 r^2 dtheta [fixed mistake with editing]
