### Rigid Body

Find the center of mass for the following bodies:
1. A paraboloid $z=a(x^2+y^2)$ between $z=0$ and $z=b$ with a uniform density $\rho$.

2. A disk of radius  $2R$ with a uniform surface density $\sigma$which has a circular hole of radius $R$ at a distance $R$ from the center of the first circle.