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Railways PYQCBT 1 2020

RRB NTPC · Question 2

Evaluate: cos 20° cos 40° cos 80°

  1. \(\frac{1}{2}\)

  2. \(\frac{1}{4}\)

  3. \(\frac{1}{8}\)

  4. \(\frac{1}{32}\)

Show answer & explanation

Answer: (C) \(\frac{1}{8}\)

Explanation

Using cos A cos(60° − A) cos(60° + A) = ¼ cos 3A with A = 20°: cos 20° cos 40° cos 80° = ¼ cos 60° = 1/8.