3 and 4 .Determinants and Matrices
hard

If $A$ is a symmetric matrix and $B$ is a skew-symmetrix matrix such that $A + B = \left[ {\begin{array}{*{20}{c}}
2&3\\
5&{ - 1}
\end{array}} \right]$ , then $AB$ is equal to

A

$\left[ {\begin{array}{*{20}{c}}
4&{ - 2}\\
1&{ - 4}
\end{array}} \right]$

B

$\left[ {\begin{array}{*{20}{c}}
4&{ - 2}\\
{ - 1}&{ - 4}
\end{array}} \right]$

C

$\left[ {\begin{array}{*{20}{c}}
{ - 4}&2\\
1&4
\end{array}} \right]$

D

$\left[ {\begin{array}{*{20}{c}}
{ - 4}&{ - 2}\\
{ - 1}&4
\end{array}} \right]$

(JEE MAIN-2019)

Solution

$A = A',B = B'$

$A + B = \left[ {\begin{array}{*{20}{c}}
2&3\\
5&{ – 1}
\end{array}} \right]\,\,\,\,\,\,\,\,….\left( 1 \right)$

$A' + B' = \left[ {\begin{array}{*{20}{c}}
2&5\\
3&{ – 1}
\end{array}} \right]\,\,\,$

$A – B = \left[ {\begin{array}{*{20}{c}}
2&5\\
3&{ – 1}
\end{array}} \right]\,\,\,\,\,\,\,\,…..\left( 2 \right)$

After addding equation $(1)$ and $(2)$

$A = \left[ {\begin{array}{*{20}{c}}
2&4\\
4&{ – 1}
\end{array}} \right]\,,B = \left[ {\begin{array}{*{20}{c}}
0&{ – 1}\\
1&0
\end{array}} \right]\,$

$AB = \left[ {\begin{array}{*{20}{c}}
4&{ – 2}\\
{ – 1}&{ – 4}
\end{array}} \right]$

Standard 12
Mathematics

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