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MCQ practice · SSC General Mathematics Board questions · Chapter 9: Trigonometric Ratio

Dhaka Board 2025

If cos²θ − sin²θ = 1, what is θ?

Think it through first, then pick an option.

  1. a 0°
  2. b 45°
  3. c 60°
  4. d 90°
Answer and explanation

Correct answer: a 0°

cos²θ − (1 − cos²θ) = 1 gives cos²θ = 1, cos θ = 1 and θ = 0°.

Why the other options are wrong

  • b 45°: At 45°, cos²θ − sin²θ = 1/2 − 1/2 = 0, not 1.
  • c 60°: At 60°, cos²θ − sin²θ = 1/4 − 3/4 = −1/2, not 1.
  • d 90°: At 90°, cos²θ − sin²θ = 0 − 1 = −1, not 1.