If A and B are two matrices such that AB = B and BA = A, then A2 + B2 =
2AB
2BA
A+B
AB
We have,A2 + B2 = AA + BB = A (BA) + B (AB) (∵ AB = B and BA = A)= (AB) A + (BA) B= BA + AB = A + B (∵ AB = B and BA = A)