A block matrix has the form $M=\left(\begin{array}{ll}A & B \\ C & D\end{array}\right)$ in which $A, B, C, D$ are matrices with respective sizes $i \times k, i \times l, j \times k, j \times l$. (a) What is the size of $M$ ?
(b) Write out the block matrix $M$ when $A=\left(\begin{array}{l}1 \\ 3\end{array}\right), B=\left(\begin{array}{rr}1 & -1 \\ 0 & 1\end{array}\right), C=\left(\begin{array}{r}1 \\ -2 \\ 1\end{array}\right), D=\left(\begin{array}{rr}1 & 3 \\ 2 & 0 \\ 1 & -1\end{array}\right)$.
(c) Show that if $N=\left(\begin{array}{cc}P & Q \\ n\end{array}\right)$ is a block matrix whose blocks have the same size as those