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Find $\mathrm{AB},$ if $\mathrm{A}=\left[\begin{array}{rr}0 & -1 \\ 0 & 2\end{array}\right]$ and $\mathrm{B}=\left[\begin{array}{ll}3 & 5 \\ 0 & 0\end{array}\right]$.
$\left[ {\begin{array}{*{20}{l}}
0&0 \\
0&0
\end{array}} \right]$
$\left[ {\begin{array}{*{20}{l}}
0&0 \\
0&0
\end{array}} \right]$
$\left[ {\begin{array}{*{20}{l}}
0&0 \\
0&0
\end{array}} \right]$
$\left[ {\begin{array}{*{20}{l}}
0&0 \\
0&0
\end{array}} \right]$
Solution
We have $A B=\left[\begin{array}{rr}0 & -1 \\ 0 & 2\end{array}\right]\left[\begin{array}{ll}3 & 5 \\ 0 & 0\end{array}\right]=\left[\begin{array}{ll}0 & 0 \\ 0 & 0\end{array}\right]$
Thus, if the product of two matrices is a zero matrix, it is not necessary that one of the matrices is a zero matrix.