Given that I3 is a unit matrix of order 3 find ... - JAMB Mathematics 2012 Question

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