## Elementary Linear Algebra 7th Edition

$3$
$A=\left[ \begin{matrix} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \\ \end{matrix} \right]$ To find the trace of a matrix calculate the sum of the main diagonal entries. $Tr(A)=1+1+1=3$