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JAMB Mathematics 1981 Question 38

Question 38

Progressions

The first term of an Arithmetic Progression is 3 and the fifth term is 9. Find the number of terms in the progression if the sum of the terms is 81.

  1. A.12
  2. B.27
  3. C.9
  4. D.4
  5. E.36.
Show the answer and explanation

Answer: C (9)

Explanation

3 + 4d = 9 gives d = 3/2. n/2 × (6 + (n − 1) × 3/2) = 81 simplifies to n² + 3n − 108 = 0, so n = 9.