JEE Main & Advanced JEE Main Paper (Held On 19 April 2014)

  • question_answer
    Let A and B be any two 3 × 3 matrices. If A is symmetric and B is skewsymmetric, then the matrix AB - BA is:     JEE Main Online Paper (Held On 19 April 2016)

    A) skewsymmetric

    B) Symmetric

    C)  neither symmetric nor skewsymmetric

    D) I or - I, where I is an identity matrix.

    Correct Answer: B

    Solution :

    Let A be symmetric matrix and B be skew symmetric matrix. \[\therefore \]\[{{A}^{T}}=A\]and \[{{B}^{T}}=-B\] Consider \[{{(AB-BA)}^{T}}={{(AB)}^{T}}-{{(BA)}^{T}}\] \[={{B}^{T}}{{A}^{T}}-{{A}^{T}}{{B}^{T}}\] \[=(-B)(A)-(A)(-B)\] \[=-BA+AB=AB-BA\] This shows AB - BA is symmetric matrix.


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