Similar to the addition of algebraic expressions or vectors, commutative law allows the addition of matrices without dependence on order. If \( A, B \in M_{m \times n} ( \mathbb{R}) \), we have that:
\( A + B = B + A \)
Consider \( A = \begin{bmatrix} 2 & -5 \\ -1 & 3 \end{bmatrix} \) and \( B = \begin{bmatrix} 3 & 5 \\ 1 & 2 \end{bmatrix} \) (Recall addition of matrices from Operations on Matrices.)
\( A + B \) = \( \begin{bmatrix} 2 & -5 \\ -1 & 3 \end{bmatrix} + \begin{bmatrix} 3 & 5 \\ 1 & 2 \end{bmatrix} \)
= \( \begin{bmatrix} 2+3 & -5+5 \\ -1+1 & 3+2 \end{bmatrix} \)
= \( \begin{bmatrix} 5 & 0 \\ 0 & 5 \end{bmatrix} \)
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\( B + A \) = \( \begin{bmatrix} 3 & 5 \\ 1 & 2 \end{bmatrix} + \begin{bmatrix} 2 & -5 \\ -1 & 3 \end{bmatrix} \)
\( \begin{bmatrix} 3+2 & 5+(-5) \\ 1+(-1) & 2+3 \end{bmatrix} \)
\( \begin{bmatrix} 5 & 0 \\ 0 & 5 \end{bmatrix} \)
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We see that \( A + B = B + A \)