Question
If $A=\left[\begin{array}{ccc}1 & 2 & -3 \\ 5 & 0 & 2 \\ 1 & -1 & 1\end{array}\right], B=\left[\begin{array}{ccc}3 & -1 & 2 \\ 4 & 2 & 5 \\ 2 & 0 & 3\end{array}\right]$ and $C=\left[\begin{array}{ccc}4 & 1 & 2 \\ 0 & 3 & 2 \\ 1 & -2 & 3\end{array}\right]$,
then compute $(A+B)$ and $(B-$ C). Also, verify that $A+(B-C)=(A+B)-C$.

Answer

$A + B = \left[ {\begin{array}{*{20}{c}} 1&2&{ - 3} \\ 5&0&2 \\ 1&{ - 1}&1 \end{array}} \right] + \left[ {\begin{array}{*{20}{c}} 3&{ - 1}&2 \\ 4&2&5 \\ 2&0&3 \end{array}} \right]$ $ = \left[ {\begin{array}{*{20}{c}} {1 + 3}&{2 - 1}&{ - 3 + 2} \\ {5 + 4}&{0 + 2}&{2 + 5} \\ {1 + 2}&{ - 1 + 0}&{1 + 3} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} 4&1&{ - 1} \\ 9&2&7 \\ 3&{ - 1}&4 \end{array}} \right]$
$B - C = \left[ {\begin{array}{*{20}{c}} 3&{ - 1}&2 \\ 4&2&5 \\ 2&0&3 \end{array}} \right] - \left[ {\begin{array}{*{20}{c}} 4&1&2 \\ 0&3&2 \\ 1&{ - 2}&3 \end{array}} \right]$ $ = \left[ {\begin{array}{*{20}{c}} {3 - 4}&{ - 1 - 1}&{2 - 2} \\ {4 - 0}&{2 - 3}&{5 - 2} \\ {2 - 1}&{0 + 2}&{3 - 3} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} { - 1}&{ - 2}&0 \\ 4&{ - 1}&3 \\ 1&2&0 \end{array}} \right]$
Now, we show; A + (B – C) = (A + B) – C
$ \Rightarrow \left[ {\begin{array}{*{20}{c}} 1&2&{ - 3} \\ 5&0&2 \\ 1&{ - 1}&1 \end{array}} \right] + \left[ {\begin{array}{*{20}{c}} { - 1}&{ - 2}&0 \\ 4&{ - 1}&3 \\ 1&2&0 \end{array}} \right]$$ = \left[ {\begin{array}{*{20}{c}} 4&1&{ - 1} \\ 9&2&7 \\ 3&{ - 1}&4 \end{array}} \right] - \left[ {\begin{array}{*{20}{c}} 4&1&2 \\ 0&3&2 \\ 1&{ - 2}&3 \end{array}} \right]$
$ \Rightarrow \left[ {\begin{array}{*{20}{c}} {1 - 1}&{2 - 2}&{ - 3 + 0} \\ {5 + 4}&{0 - 1}&{2 + 3} \\ {1 + 1}&{ - 1 + 2}&{1 + 0} \end{array}} \right]$$ = \left[ {\begin{array}{*{20}{c}} {4 - 4}&{1 - 1}&{ - 1 - 2} \\ {9 - 0}&{2 - 3}&{7 - 2} \\ {3 - 1}&{ - 1 + 2}&{4 - 3} \end{array}} \right]$
$ \Rightarrow \left[ {\begin{array}{*{20}{c}} 0&0&{ - 3} \\ 9&{ - 1}&5 \\ 2&1&1 \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} 0&0&{ - 3} \\ 9&{ - 1}&5 \\ 2&1&1 \end{array}} \right]$
$\Rightarrow$ L.H.S. = R.H.S. Hence Proved.

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