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NCERT Exemplar · Q61

Q.If AA and BB are matrices of same order, then (AB′−BA′)(AB' - BA') is a
(A) skew symmetric matrix
(B) null matrix
(C) symmetric matrix
(D) unit matrix

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The expression (AB′−BA′)(AB' - BA') is always a skew symmetric matrix for any two conformable matrices AA and BB, because its transpose equals its own negative.

Why This Works: The Core Idea

The problem asks about the nature of the matrix M=AB′−BA′M = AB' - BA', where AA and BB are of the same order. The key is to check whether MM is symmetric (M′=MM' = M) or skew symmetric (M′=−MM' = -M).

The transpose operation has a beautiful property: it reverses the order of a product. So (AB′)′=(B′)′A′=BA′(AB')' = (B')' A' = BA'. This reversal is the engine behind the entire proof.

Tip

Whenever you see an expression of the form X−X′X - X', it is always skew symmetric. Here, X=AB′X = AB', so X′=BA′X' = BA', and M=X−X′M = X - X' is automatically skew symmetric. No need to expand further — just recognize the pattern.

Step-by-Step Reasoning

1. Write down the given matrix.

Let M=AB′−BA′M = AB' - BA'. We need to find M′M', the transpose of MM.

2. Take the transpose of MM.

Using the property that (P−Q)′=P′−Q′(P - Q)' = P' - Q', we get:

M′=(AB′)′−(BA′)′M' = (AB')' - (BA')'

3. Apply the reversal rule for transposes.

For any two matrices XX and YY, (XY)′=Y′X′(XY)' = Y' X'. So:

(AB′)′=(B′)′A′=BA′(AB')' = (B')' A' = B A'

because (B′)′=B(B')' = B. Similarly:

(BA′)′=(A′)′B′=AB′(BA')' = (A')' B' = A B'

4. Substitute back.

M′=BA′−AB′M' = B A' - A B'

5. Compare M′M' with MM. …

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