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NDA 2025 · PYQ · Matrices / Orthogonal · medium

If A = [x y z; y z x; z x y] where x, y, z are integers, is an orthogonal matrix, then what is A² equal to?

  1. A.Null matrix
  2. B.Identity matrix✓ Correct
  3. C.A
  4. D.-A

Explanation

For an orthogonal matrix, A·Aᵀ = I. Notice that A given here is symmetric (because the matrix [x y z; y z x; z x y] — checking: A[1][2]=y, A[2][1]=y ✓; A[1][3]=z, A[3][1]=z ✓; A[2][3]=x, A[3][2]=x ✓). So A = Aᵀ. Hence A² = A·A = A·Aᵀ = I, the identity matrix.
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