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JAMB Mathematics 2006 Question 50

Question 50

Progressions

How many terms of the series 3, −6, +12, −24, +... are needed to make a total of 1 − 28?

  1. A.12
  2. B.10
  3. C.9
  4. D.8
Show the answer and explanation

Answer: D (8)

Explanation

a = 3 and r = −2, so Sn = 3(1 − (−2)n)/(1 + 2) = 1 − (−2)n. This equals 1 − 28 when n = 8.