The Trace(A) command computes the sum of the diagonal elements of Matrix A.

$\mathrm{with}\left(\mathrm{Student}\left[\mathrm{LinearAlgebra}\right]\right)\:$

$A≔⟨⟨a\,b\,c⟩\left|⟨d\,e\,f⟩\right|⟨g\,h\,i⟩⟩$

$A≔\left[\begin{array}{ccc}a& d& g\\ b& e& h\\ c& f& i\end{array}\right]$

$\mathrm{Trace}\left(A\right)$

$a+e+i$