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WAEC Further Mathematics 2021 Question 25

Question 25

Trigonometry

If sin x = 12/13 and sin y = 4/5, where x and y are both acute angles, find cos(x + y).

  1. A.48/65
  2. B.13/65
  3. C.−33/65
  4. D.−48/65
Show the answer and explanation

Answer: C (−33/65)

Explanation

cos x = 5/13 and cos y = 3/5. cos(x + y) = (5/13)(3/5) − (12/13)(4/5) = 15/65 − 48/65 = −33/65.