MCQ
If $A$ and $B$ are two matrices such that $AB = B$ and $BA = A, A^2 + B^2$ is equal to$:$
  • A
    $2AB$
  • B
    $2BA$
  • $A + B$
  • D
    $AB$

Answer

Correct option: C.
$A + B$
Given $AB = A$ and $BA = B,$ then
$\Rightarrow BAB = B^2$ and $ABA = A^2$
$\Rightarrow BA = B^2$ and $AB = A^2$
$\Rightarrow B = B^2$ and $A = A^2$
$\Rightarrow A^2 + B^2 = A + B$

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