Chapter 1.SE, Problem 9SE

Find $A^{-1}$ for each of the following matrices $A$

a) $A=\left[\begin{array}{ccc}1& 2& 1\\ 2& 5& 4\\ 1& 1& 0\end{array}\right]$

b) $A=\left[\begin{array}{cc}\cos \theta & -\sin \theta \\ \sin \theta & \cos \theta \end{array}\right]$