Question
Let $A$ and $B$ be square matrices of the same order. Does $(A + B)^2 = A^2 + 2AB + B^2$ hold? If not, why?

Answer

$\text{LHS} = (A + B)^2$
$= (A + B)(A + B)$
$= A(A + B) + B(A + B)$
$= A^2 + AB + BA + B^2$
We know that a matrix does not have commutative property.
So, $AB \neq BA$
Thus, $(A + B)^2 \neq A^2 + 2AB + B^2$

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