a
Let $A=\left[\begin{array}{ccc}1 & 0 & 0 \\ 0 & \cos a & \sin a \\ 0 & \sin a & -\cos a\end{array}\right]$ $a$
We have,
$|A|=1\left(-\cos ^{2} a-\sin ^{2} a\right)=-\left(\cos ^{2} a+\sin ^{2} a\right)=-1$
Now,
$A_{11}=-\cos ^{2} a-\sin ^{2} a=-1, A_{12}=0, A_{13}=0$
$A_{21}=0, A_{22}=-\cos a, A_{23}=-\sin a$
$A_{31}=0, A_{32}=-\sin a, A_{33}=\cos a$
$\therefore a d j A=\left[\begin{array}{ccc}-1 & 0 & 0 \\ 0 & -\cos a & -\sin a \\ 0 & -\sin a & \cos a\end{array}\right]$
$\therefore A^{-1}=\frac{1}{|A|} \operatorname{adj} A=-1\left[\begin{array}{ccc}-1 & 0 & 0 \\ 0 & -\cos a & -\sin a \\ 0 & -\sin a & \cos a\end{array}\right]=\left[\begin{array}{ccc}1 & 0 & 0 \\ 0 & \cos a & \sin a \\ 0 & \sin a & -\cos a\end{array}\right]$