Question
Let $$A$$ and $$B$$ be two symmetric matrices of order 3.
Statement - 1 : $$A(BA)$$ and $$(AB)A$$ are symmetric matrices.
Statement - 2 : $$AB$$ is symmetric matrix if matrix multiplication of $$A$$ with $$B$$ is commutative.
A.
Statement - 1 is true, Statement - 2 is true; Statement - 2 is not a correct explanation for Statement - 1.
B.
Statement - 1 is true, Statement - 2 is false.
C.
Statement - 1 is false, Statement - 2 is true.
D.
Statement - 1 is true, Statement - 2 is true; Statement - 2 is a correct explanation for Statement - 1.
Answer :
Statement - 1 is true, Statement - 2 is true; Statement - 2 is not a correct explanation for Statement - 1.
Solution :
$$\eqalign{
& \therefore \,\,A' = A,\,\,B' = B \cr
& {\text{Now }}\left( {A\left( {BA} \right)} \right)' = \left( {BA} \right)'A' \cr
& = \left( {A'B'} \right)A' = \left( {AB} \right)A = A\left( {BA} \right) \cr
& {\text{Similarly}}\,\,\left( {\left( {AB} \right)A} \right)' = \left( {AB} \right)A \cr} $$
So, $$A(BA)$$ and $$(AB)A$$ are symmetric matrices.
Again $$(AB)' = B'A’ = BA$$
Now if $$BA = AB,$$ then $$AB$$ is symmetric matrix.