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Exercise 3.3 · Q8

Q.For the matrix A=[1567]A = \begin{bmatrix} 1 & 5 \\ 6 & 7 \end{bmatrix}, verify that

(i) (A+A′)(A + A') is a symmetric matrix
(ii) (A−A′)(A - A') is a skew symmetric matrix
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For any square matrix AA, the sum A+A′A + A' is always symmetric and the difference A−A′A - A' is always skew-symmetric. For A=[1567]A = \begin{bmatrix} 1 & 5 \\ 6 & 7 \end{bmatrix}, we compute A+A′=[2111114]A + A' = \begin{bmatrix} 2 & 11 \\ 11 & 14 \end{bmatrix} (symmetric) and A−A′=[0−110]A - A' = \begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix} (skew-symmetric), verifying both properties.

The Core Idea

This problem is not about doing random matrix arithmetic — it's about seeing a beautiful structural fact. Every square matrix can be split into two special parts: a symmetric part and a skew-symmetric part. The symmetric part is (A+A′)/2(A + A')/2 and the skew-symmetric part is (A−A′)/2(A - A')/2. Here, we're just checking the numerators before halving.

Why does this work? Because transposition flips the roles. When you add AA and A′A', the off-diagonal entries add up to the same number in symmetric positions — that's exactly what symmetry means. When you subtract, the off-diagonals become negatives of each other, which is the definition of skew-symmetry.

For any square matrix AA:

  • A+A′A + A' is always symmetric: (A+A′)′=A′+A=A+A′(A + A')' = A' + A = A + A'
  • A−A′A - A' is always skew-symmetric: (A−A′)′=A′−A=−(A−A′)(A - A')' = A' - A = -(A - A')

Step-by-Step Verification

1. Write down AA and find A′A'

We have:

A=[1567]A = \begin{bmatrix} 1 & 5 \\ 6 & 7 \end{bmatrix}

The transpose swaps rows and columns:

A′=[1657]A' = \begin{bmatrix} 1 & 6 \\ 5 & 7 \end{bmatrix}

Notice how the off-diagonal entries 55 and 66 have swapped places.

2. Compute A+A′A + A'

Add entry by entry:

A+A′=[1+15+66+57+7]=[2111114]A + A' = \begin{bmatrix} 1+1 & 5+6 \\ 6+5 & 7+7 \end{bmatrix} = \begin{bmatrix} 2 & 11 \\ 11 & 14 \end{bmatrix}

Now check symmetry: a matrix is symmetric if it equals its own transpose. Look at the off-diagonals — both are 1111. The transpose of this matrix is:

(A+A′)′=[2111114]′=[2111114](A + A')' = \begin{bmatrix} 2 & 11 \\ 11 & 14 \end{bmatrix}' = \begin{bmatrix} 2 & 11 \\ 11 & 14 \end{bmatrix}

It's identical to the original. So A+A′A + A' is symmetric. Done.

Tip

You don't even need to compute the transpose explicitly. For a 2×22 \times 2 matrix, symmetry just means the top-right equals the bottom-left. Here 11=1111 = 11, so it's symmetric.

3. Compute A−A′A - A'

Subtract entry by entry:

A−A′=[1−15−66−57−7]=[0−110]A - A' = \begin{bmatrix} 1-1 & 5-6 \\ 6-5 & 7-7 \end{bmatrix} = \begin{bmatrix} 0 & -1 \\ 1 & 0 \end{bmatrix}

Now check skew-symmetry: a matrix is skew-symmetric if its transpose equals its negative. Take the transpose: …

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