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JAMB Mathematics 1984 Question 29

Question 29

Trigonometry

The sides of a triangle are (x + 4)cm, x cm and (x − 4)cm respectively. If the cosine of the largest angle is 1/5, find the value of x.

  1. A.24cm
  2. B.20cm
  3. C.28cm
  4. D.88/7cm
  5. E.0cm
Show the answer and explanation

Answer: A (24cm)

Explanation

The largest angle faces x + 4. Cosine rule: (x + 4)² = x² + (x − 4)² − 2x(x − 4)/5. Multiplying by 5 and simplifying gives 3x² − 72x = 0, so x = 24.