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Miscellaneous Exercise · Q10

Q.If the matrix AA is both symmetric and skew symmetric, then (A) AA is a diagonal matrix (B) AA is a zero matrix (C) AA is a square matrix (D) None of these

CBSENCERTSubjective· 1mImportance★★★★★
Appeared in past exams:CBSE 2025· Set 65/2/1· 1mexact
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A matrix that is both symmetric and skew-symmetric must satisfy A=ATA = A^T and A=−ATA = -A^T, which forces every element to be zero. Therefore, AA is necessarily the zero matrix.

The key here is to understand what it means for a matrix to be symmetric and skew-symmetric at the same time. These are two different properties that usually describe opposite kinds of matrices — one is equal to its transpose, the other is equal to the negative of its transpose. When both hold simultaneously, the only possible matrix is the zero matrix.

Let’s break it down step by step.

  1. Recall the definitions.

    A square matrix AA is symmetric if A=ATA = A^T. That means aij=ajia_{ij} = a_{ji} for all i,ji, j.

    A square matrix AA is skew-symmetric if A=−ATA = -A^T. That means aij=−ajia_{ij} = -a_{ji} for all i,ji, j.

  2. Apply both conditions together.

    If AA is both symmetric and skew-symmetric, then for every entry aija_{ij} we have:

aij=aji(from symmetry)a_{ij} = a_{ji} \quad \text{(from symmetry)}

and also

aij=−aji(from skew-symmetry).a_{ij} = -a_{ji} \quad \text{(from skew-symmetry)}.

  1. Combine the two equations. From the two equalities, we get:

aij=−aij.a_{ij} = -a_{ij}.

Adding aija_{ij} to both sides gives:

2aij=0⇒aij=0.2a_{ij} = 0 \quad \Rightarrow \quad a_{ij} = 0.

This holds for every ii and jj, meaning every entry of AA is zero.

  1. What about diagonal entries? For i=ji = j, the skew-symmetric condition says aii=−aiia_{ii} = -a_{ii}, which also forces aii=0a_{ii} = 0. So even the diagonal is all zeros. …

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