Prove that (AB)' is not same as (BA)'.
Answers
Answer:
Let A be m×n. Since AB and BA both exist, hence B must be n×m. Thus, AB is m×m and BA is n×n. So far, all we have established is both AB and BA are square matrices. If m≠n then AB and BA have different sizes. They can’t be equal. If m=n then AB and BA can be compared but in general AB≠BA since matrix multiplication is non-commutative in general. In the special case that A and B commute with each other, you will have AB=BA.
Two matrices are said to commute if AB=BA or equivalently, their commutator [A,B]=AB−BA is 0. There is no straightforward answer to prove if the matrices commute. However, there are some special examples which may be relevant for you. We are assuming here that both A and B are square matrices with same dimensions. Identity matrix commutes with every matrix . A diagonal matrix commutes with every matrix. If A and B are both symmetric and their product AB is also symmetric then AB=BA.