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Which of the following are correct?

(A) If A and B are symmetric matrices such that AB = BA, then AB is symmetric.

(B) If A and B are symmetric matrices of the same order, then (A+B) is a symmetric matrix.

(C) If A and B are symmetric matrices of the same order, then (AB-BA) is a symmetric matix.

(D) If A and B are symmetric matrices of the same order, then (AB+BA) is a skew symmetric matrix

Choose the correct answer from the options given below:

Solution

✅ Correct Option: 1

A matrix is symmetric if it equals its own transpose: AT=AA^T = A

For symmetric matrices A and B: AT=AA^T = A and BT=BB^T = B

Transpose rule: (AB)T=BTAT(AB)^T = B^T A^T


Statement (A): If AB = BA and both are symmetric, then AB is symmetric.

(AB)T=BTAT(AB)^T = B^T A^T

=BA= BA

=AB= AB (given that AB = BA)

Therefore (AB)T=AB(AB)^T = AB, so AB is symmetric.

Statement (A) is correct.


Statement (B): (A+B) is symmetric.

(A+B)T=AT+BT(A+B)^T = A^T + B^T

=A+B= A + B

Therefore (A+B)T=(A+B)(A+B)^T = (A+B), so (A+B) is symmetric.

Statement (B) is correct.


Statement (C): (AB-BA) is symmetric.

(AB−BA)T=(AB)T−(BA)T(AB-BA)^T = (AB)^T - (BA)^T

=BTAT−ATBT= B^T A^T - A^T B^T

=BA−AB= BA - AB

=−(AB−BA)= -(AB - BA)

Therefore (AB−BA)T=−(AB−BA)(AB-BA)^T = -(AB-BA), so (AB-BA) is skew-symmetric, not symmetric.

Statement (C) is incorrect.


Statement (D): (AB+BA) is skew-symmetric.

(AB+BA)T=(AB)T+(BA)T(AB+BA)^T = (AB)^T + (BA)^T

=BTAT+ATBT= B^T A^T + A^T B^T

=BA+AB= BA + AB

=AB+BA= AB + BA

Therefore (AB+BA)T=(AB+BA)(AB+BA)^T = (AB+BA), so (AB+BA) is symmetric, not skew-symmetric.

Statement (D) is incorrect.


Only statements (A) and (B) are correct.

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