Recall that a unit matrice is a diagonal matrix in which the elements in the leading diagonal is unity. Therefore, I3 = \(\begin{pmatrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{pmatrix}\) I3 = \(+1\begin{vmatrix} 1 & 0 \\ 0 & 1 \end{vmatrix} - 0\begin{vmatrix} 0 & 0 \\ 0 & 1 \end{vmatrix} + 0 \begin{vmatrix} 0 & 1 \\ 0 & 0 \end{vmatrix} \) I3 = +1(1 - 0) - 0(0 - 0) + 0(0 - 0) = 1(1) = 1