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Let \(A = \begin{pmatrix} 1 & 3 & 5 \\ -6 & 8 & 3 \\ -4 & 6 & 5 \end{pmatrix}\) and \(P = \frac{1}{2}(A + A^{T})\). Then

  • A.\(P^{T} = P\)
  • B.\(P^{T} = -P\)
  • C.\(P^{T} = 2P\)
  • D.\(P^{T} = -2P\)
  • E.\(P^{T} = 3P\)

Answer: A

The relation is \(P^{T} = P\).

Transposition distributes over a sum and leaves a scalar factor alone, and \((A^{T})^{T} = A\).

\[ P^{T} = \frac{1}{2}(A^{T} + A) = P \]

So P is the symmetric part of A, whatever A is; the entries of A play no role. \(P^{T} = -P\) describes the skew-symmetric part \(\frac{1}{2}(A - A^{T})\), and the multiples 2P, -2P and 3P cannot hold for a non-zero P.

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