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NDA 2024 · PYQ · Matrices · medium

If A = [[sin2θ, 2sin²θ – 1, 0],[cos2θ, 2sinθ cosθ, 0],[0, 0, 1]], then which of the following statements is/are correct? 1. A⁻¹ = adj A 2. A is skew-symmetric matrix 3. A⁻¹ = Aᵀ. Select the correct answer using the code given below:

  1. A.1 only
  2. B.1 and 2
  3. C.1 and 3✓ Correct
  4. D.2 and 3

Explanation

Note 2sin²θ – 1 = –cos2θ and 2sinθcosθ = sin2θ. So A = [[sin2θ, –cos2θ, 0],[cos2θ, sin2θ, 0],[0,0,1]]. This is an orthogonal matrix (rotation matrix in 2D extended). det(A) = sin²2θ + cos²2θ = 1. For orthogonal matrices with det = 1, A⁻¹ = Aᵀ. Also adj A = det(A)·A⁻¹ = A⁻¹. A is not skew-symmetric since diagonal entries are non-zero. So statements 1 and 3 are correct.
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