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WAEC Further Mathematics 2014 Question 10

Question 10

Sequences and Series

If 2, (k + 1), 8, … form an exponential sequence (GP), find the values of k.

  1. A.−3 and 5
  2. B.5 and −5
  3. C.3 and −3
  4. D.−5 and 3
Show the answer and explanation

Answer: D (−5 and 3)

Explanation

(k + 1)/2 = 8/(k + 1), so (k + 1)² = 16, k + 1 = ±4 and k = 3 or −5.