Question
Let \[A = \left[ {\begin{array}{*{20}{c}}
5&6&1\\
2&{ - 1}&5
\end{array}} \right].\] Let there exist a matrix $$B$$ such that \[AB = \,\left[ {\begin{array}{*{20}{c}}
{35}&{49}\\
{29}&{13}
\end{array}} \right].\] What is $$B$$ equal to ?
A.
\[\left[ {\begin{array}{*{20}{c}}
5&1&4\\
2&{ 6}&3
\end{array}} \right]\]
B.
\[\left[ {\begin{array}{*{20}{c}}
2&6&3\\
5&{1}&4
\end{array}} \right]\]
C.
\[\left[ {\begin{array}{*{20}{c}}
5\\
1\\
4
\end{array}\,\,\,\,\,\begin{array}{*{20}{c}}
2\\
6\\
3
\end{array}} \right]\]
D.
\[\left[ {\begin{array}{*{20}{c}}
2\\
6\\
3
\end{array}\,\,\,\,\,\begin{array}{*{20}{c}}
5\\
1\\
4
\end{array}} \right]\]
Answer :
\[\left[ {\begin{array}{*{20}{c}}
5\\
1\\
4
\end{array}\,\,\,\,\,\begin{array}{*{20}{c}}
2\\
6\\
3
\end{array}} \right]\]
Solution :
\[\begin{array}{l}
A = \left[ {\begin{array}{*{20}{c}}
5&6&1\\
2&{ - 1}&5
\end{array}} \right]\,\,{\rm{and}}\,{\rm{let}}\,\,B = \left[ {\begin{array}{*{20}{c}}
5\\
1\\
4
\end{array}\,\,\,\,\,\begin{array}{*{20}{c}}
2\\
6\\
3
\end{array}} \right]\\
\therefore \,AB = \left[ {\begin{array}{*{20}{c}}
5&6&1\\
2&{ - 1}&5
\end{array}} \right]\,\left[ {\begin{array}{*{20}{c}}
5\\
1\\
4
\end{array}\,\,\,\,\,\begin{array}{*{20}{c}}
2\\
6\\
3
\end{array}} \right]\\
= \,\left[ {\begin{array}{*{20}{c}}
{25 + 6 + 4}&{10 + 36 + 3}\\
{10 - 1 + 20}&{4 - 6 + 15}
\end{array}} \right]\\
= \,\left[ {\begin{array}{*{20}{c}}
{35}&{49}\\
{29}&{13}
\end{array}} \right]
\end{array}\]