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JAMB Mathematics 1988 Question 28

Question 28

Progressions

If the sum of the 8th and 9th terms of an arithmetic progression is 72 and the 4th term is −6, find the common difference

  1. A.4
  2. B.8
  3. C.6⅔
  4. D.9⅓
Show the answer and explanation

Answer: D (9⅓)

Explanation

(a + 7d) + (a + 8d) = 72 gives 2a + 15d = 72, and a + 3d = −6 gives 2a + 6d = −12. Subtracting, 9d = 84, so d = 9⅓.