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NCERT Exemplar · Q74

Q.If AA and BB are symmetric matrices, then

(i) AB−BAAB - BA is a _________.
(ii) BA−2ABBA - 2AB is a _________.
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For symmetric matrices AA and BB, the commutator AB−BAAB - BA is always skew-symmetric, while BA−2ABBA - 2AB is neither symmetric nor skew-symmetric in general — it is simply an arbitrary matrix.

Why This Approach Works

The key idea is simple: a symmetric matrix equals its own transpose, and a skew-symmetric matrix equals the negative of its transpose. When you multiply symmetric matrices, the transpose of the product reverses the order: (AB)T=BTAT=BA(AB)^T = B^T A^T = BA (since AT=AA^T = A, BT=BB^T = B). So the transpose of ABAB is BABA, not ABAB itself. This reversal is the engine behind the entire problem.

We are asked to classify two expressions: AB−BAAB - BA and BA−2ABBA - 2AB. The first is a classic commutator — its behaviour under transpose is well-known. The second is a linear combination that does not have a standard name. We will check each by taking its transpose and comparing with the original.

Watch out

A common mistake is to assume ABAB is symmetric just because AA and BB are. That is false unless AA and BB commute. Always remember: (AB)T=BTAT(AB)^T = B^T A^T, not ATBTA^T B^T.


Step-by-Step Solution

1. Recall the definitions

A matrix MM is symmetric if MT=MM^T = M.

A matrix MM is skew-symmetric if MT=−MM^T = -M.

Given: AT=AA^T = A and BT=BB^T = B.

2. Transpose of a product

For any matrices of compatible sizes:

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

Since AA and BB are symmetric, this becomes:

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

Tip

This is the only fact you need. The transpose flips the order, and symmetry removes the transposes on the individual matrices.

3. Analyse AB−BAAB - BA

Let C=AB−BAC = AB - BA. Take its transpose:

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

But BA−AB=−(AB−BA)=−CBA - AB = -(AB - BA) = -C. So:

CT=−CC^T = -C

This is exactly the definition of a skew-symmetric matrix.

For symmetric AA and BB, the commutator AB−BAAB - BA is always skew-symmetric:

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

Thus, the first blank is skew-symmetric.

4. Analyse BA−2ABBA - 2AB

Let D=BA−2ABD = BA - 2AB. Take its transpose:

DT=(BA−2AB)T=(BA)T−2(AB)T=AB−2BAD^T = (BA - 2AB)^T = (BA)^T - 2(AB)^T = AB - 2BA

Now compare DTD^T with DD:

  • For DD to be symmetric, we would need DT=DD^T = D, i.e. AB−2BA=BA−2ABAB - 2BA = BA - 2AB.
  • For DD to be skew-symmetric, we would need DT=−DD^T = -D, i.e. AB−2BA=−(BA−2AB)=2AB−BAAB - 2BA = -(BA - 2AB) = 2AB - BA. …

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