show that the matrix B'AB is symmetric or skew symmetric according as A is symmetric or skew symmetric.?
Case I
when A is symmetric, i.e, A'=A
(B'AB)' = (AB)'(B')'
=B'A'B
but because A'=A
this implies, (B'AB)'=B'AB
i.e, it is symmetric.
Case II
when A is skew symmetric, i.e, A'=-A
(B'AB)'= (AB)'(B')'
=B'A'B
but A'=-A
this implies, (B'AB)'=-B'AB
i.e, it is skew symmetric.
Hence, matrix B'AB is symmetric or skew symmetric according as A is symmetric or skew symmetric.
when A is symmetric, i.e, A'=A
(B'AB)' = (AB)'(B')'
=B'A'B
but because A'=A
this implies, (B'AB)'=B'AB
i.e, it is symmetric.
Case II
when A is skew symmetric, i.e, A'=-A
(B'AB)'= (AB)'(B')'
=B'A'B
but A'=-A
this implies, (B'AB)'=-B'AB
i.e, it is skew symmetric.
Hence, matrix B'AB is symmetric or skew symmetric according as A is symmetric or skew symmetric.