11 If A, B are symmetric matrices of same order, then AB – BA is a
(A) Skew symmetric matrix
(B) Symmetric matrix
(C) Zero matrix
(D) Identity matrix
12 If 
cos
sin
A
,
sin
cos
α
−
α


= 

α
α


and  A + A′ = I, then the value of α is
(A) 6
π
(B) 3
π
(C) π
(D) 3
2
π
3 7  Invertible Matrices
Definition 6 If A is a square matrix of order m, and if there exists another square
matrix B of the same order m, such that AB = BA = I, then B is called the inverse
matrix of A and it is denoted by A– 1