For vectors, we will use the notation $[]$k to extract the kth entry, e$.$g$.,[$Ab$]$k denotes the kth

entry of the vector Ab$.$ Similarly, for matrices, we will use $[]$kl to extract the kth row and lth

column. Write the following sums using the vector a in Rn$,$ the matrices B$,$ C in Rn$\backslash$times n$,$ and

these new operators. The first problem is solved as an example, showing several solutions.