(a) Let us consider a lamina of an arbitrary shape and indicate by 1,2 and 3, three axes coinciding with x, y and z-axes and the plane of lamina as x - y plane.
(b) Let us take the plane of the disc as x - y plane and origin to the centre of the disc (Fig.) From the symmetry Ix = Iy . Let us consider a ring element of radius r and thickness dr, then the moment of inertia of the ring element about the y-axis.