$($a$)$ Use Matlab to compute $AC,A^{-1}C$ and $C^{T}A{C}^{-1}$ if exists.

$($b$)$ Use Matlab to test whether $CD=DC$ and $(CD{)}^{-1}=C^{-1}{D}^{-1}.$

$($c$)$ Compute $(AD{)}^{20}.$