If A   is a 3 × 3 non-singular matrix such that A A ' = A ' A and B = A - 1 A ' , then…

If A is a 3×3 non-singular matrix such that AA'=A'A and B=A-1A', then BB' equals, where X' denotes the transpose of the matrix X.
  1. B-1
  2. B-1'
  3. I+B
  4. I

Solution

A'=AT

AA'=A'A

B=A-1A'

B'=A-1A''=A''·A-1'=AA-1'

BB'=A-1A'·AA-1'

       =A-1·A·A'·A-1'     ∵  AA'=A'A

       =I·A'·A-1'

       =I·A-1A'           ∵  CD'=D'C'

       =I·I'

       =I2                                      ∵  I=I'

       =I

Asked in: JEE Main 2014 (06 Apr)

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