Answer:
q.e.d.
Step-by-step explanation:
since AB and BA both exist, then A and B both have same size. since AB has main diagonal entry product of i-th row of A and i-th colom of B while BA has main diag entry product of i-th row of B and i-th colom of A, then tr(AB) = tr(BA) by using some algebraic manipulations. substitute B := BC to get tr(ABC) = tr(BCA)