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JAMB Mathematics 1988 Question 42

Question 42

Trigonometry

In triangle PQR, PQ = 1cm, QR = 2cm and PQ̂R = 120°. Find the longest side of the triangle.

  1. A.√3cm
  2. B.√7cm
  3. C.3cm
  4. D.7cm
Show the answer and explanation

Answer: B (√7cm)

Explanation

The longest side is PR, opposite the 120° angle. By the cosine rule PR² = 1² + 2² − 2(1)(2)cos 120° = 5 + 2 = 7, so PR = √7cm.