Misc 2 - Show that matrix B'AB is symmetric or skew symmetric - Miscellaneous

part 2 - Misc 2 - Miscellaneous - Serial order wise - Chapter 3 Class 12 Matrices
part 3 - Misc 2 - Miscellaneous - Serial order wise - Chapter 3 Class 12 Matrices part 4 - Misc 2 - Miscellaneous - Serial order wise - Chapter 3 Class 12 Matrices

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Misc 2 Show that the matrix B’AB is symmetric or skew symmetric according as A is symmetric or skew symmetric. We need to prove B’AB is symmetric if A is symmetric and B’AB is skew symmetric if A is skew symmetric Proving B’AB is symmetric if A is symmetric Let A be a symmetric matrix, then A’ = A Taking (B’AB)’ Let AB = P = (B’P)’ = P’ (B’)’ = P’ B Putting P = AB = (AB)’ (B) = B’A’ (B) = B’AB ∴ (B’AB)’ = B’AB Thus, B’AB is a symmetric matrix Proving B’AB is skew-symmetric if A is skew-symmetric Let A be a skew-symmetric matrix, then A’ = – A Taking (B’AB)’ Let AB = P = (B’P)’ = P’ (B’)’ = P’ B Putting P = AB = (AB)’ (B) = B’A’ (B) = B’(–A)B = – B’AB ∴ (B’AB)’ = – B’AB Thus, B’AB is a skew symmetric matrix Hence, matrix B’AB is symmetric or skew symmetric according as A is symmetric or skew symmetric.

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