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WAEC Mathematics 1994 Question 11

Question 11

Progressions

If the 2nd and 5th terms of a G.P. are 6 and 48 respectively, find the sum of the first four terms.

  1. A.−45
  2. B.−15
  3. C.15
  4. D.33
  5. E.45
Show the answer and explanation

Answer: E (45)

Explanation

ar = 6 and ar⁴ = 48, so r³ = 8, r = 2 and a = 3. Sum of 4 terms = 3(2⁴ − 1)/(2 − 1) = 45.