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Matrices and Determinants question

2011 · Shift 0 · Q42
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  5. /2011 · Shift 0 · Q42

Matrices and Determinants question

2011 · Shift 0 · Q42

JEE MainMathematicsMatrices and DeterminantsMCQ+4 / −1
Let AAA and BBB be two symmetric matrices of order 333. Statement - 1 : A(BA)A(BA)A(BA) and (AB)A(AB)A(AB)A are symmetric matrices. Statement - 2 : ABABAB is symmetric matrix if matrix multiplication of AAA with BBB is commutative.
  1. A
    statement - 1 is true, statement - 2 is true; statement - 2 is not a correct explanation for statement - 1.
  2. B
    statement - 1 is true, statement - 2 is false.
  3. C
    statement - 1 is false, statement -2 is true
  4. D
    statement -1 is true, statement - 2 is true; statement - 2 is a correct explanation for statement - 1.
View written solutionFree

Correct answer: A

  1. Given:

    • AAA and BBB are symmetric matrices of order 333.
    • So, AT=A,BT=B.A^T=A,\qquad B^T=B.AT=A,BT=B.
  2. Statement–1: A(BA)A(BA)A(BA) and (AB)A(AB)A(AB)A are symmetric matrices

    First note that matrix multiplication is associative, so A(BA)=(AB)A=ABA.A(BA)=(AB)A=ABA.A(BA)=(AB)A=ABA.

    Now check symmetry of ABAABAABA: (ABA)T=ATBTAT.(ABA)^T = A^T B^T A^T.(ABA)T=ATBTAT. Since AAA and BBB are symmetric, (ABA)T=ABA.(ABA)^T = ABA.(ABA)T=ABA. Hence ABAABAABA is symmetric.

    Therefore both A(BA)A(BA)A(BA) and (AB)A(AB)A(AB)A are symmetric matrices.

    So, Statement–1 is true.

  3. Statement–2: ABABAB is symmetric if matrix multiplication of AAA with BBB is commutative

    If multiplication is commutative, then AB=BA.AB=BA.AB=BA.

    Now, (AB)T=BTAT=BA.(AB)^T = B^T A^T = BA.(AB)T=BTAT=BA. Since AB=BAAB=BAAB=BA, (AB)T=AB.(AB)^T = AB.(AB)T=AB. Hence ABABAB is symmetric.

    So, Statement–2 is true.

  4. Does Statement–2 explain Statement–1?

    Statement–2 says: if AB=BAAB=BAAB=BA, then ABABAB is symmetric.

    But Statement–1 concerns the matrix ABA,ABA,ABA, and its symmetry follows directly from (ABA)T=ATBTAT=ABA,(ABA)^T = A^T B^T A^T = ABA,(ABA)T=ATBTAT=ABA, without needing AB=BAAB=BAAB=BA.

    So Statement–2 is not the correct explanation of Statement–1.

  5. Conclusion

    • Statement–1: True
    • Statement–2: True
    • Statement–2 is not the correct explanation for Statement–1.

    Therefore, the correct option is A.

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