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If A and B are symmetric matrices of order 3 x 3 then the matrix 2AB−BA2AB - BA is:

Solution

✅ Correct Option: 4

Given: A and B are symmetric matrices of order 3 x 3.

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

For any matrices: (XY)T=YTXT(XY)^T = Y^T X^T


Let M=2AB−BAM = 2AB - BA

Taking transpose:

MT=(2AB−BA)TM^T = (2AB - BA)^T

MT=(2AB)T−(BA)TM^T = (2AB)^T - (BA)^T

MT=2(AB)T−(BA)TM^T = 2(AB)^T - (BA)^T


Applying the transpose property:

MT=2BTAT−ATBTM^T = 2B^T A^T - A^T B^T

Since AT=AA^T = A and BT=BB^T = B:

MT=2BA−ABM^T = 2BA - AB


For M to be symmetric, we need MT=MM^T = M:

2BA−AB=2AB−BA2BA - AB = 2AB - BA

3BA=3AB3BA = 3AB

BA=ABBA = AB

This is not true in general since matrix multiplication is not commutative.

Therefore, M is not symmetric.


For M to be skew-symmetric, we need MT=−MM^T = -M:

2BA−AB=−(2AB−BA)2BA - AB = -(2AB - BA)

2BA−AB=−2AB+BA2BA - AB = -2AB + BA

2BA−AB=BA−2AB2BA - AB = BA - 2AB

BA=−ABBA = -AB

This is also not true in general.

Therefore, M is not skew-symmetric.


The matrix 2AB−BA2AB - BA is neither symmetric nor skew-symmetric.

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