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JAMB Mathematics 1978 Question 28

Question 28

Progressions

An arithmetic progression has first term 11 and fourth term 32. The sum of the first nine terms is

  1. A.351
  2. B.531
  3. C.135
  4. D.315
  5. E.153.
Show the answer and explanation

Answer: A (351)

Explanation

11 + 3d = 32 gives d = 7. S₉ = 9/2 × (2 × 11 + 8 × 7) = 9/2 × 78 = 351.