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JAMB Mathematics 1999 Question 20

Question 20

Progressions

The first term of a geometrical progression is twice its common ratio. Find the sum of the first two terms of the progression if its sum to infinity is 8.

  1. A.8/5
  2. B.8/3
  3. C.72/25
  4. D.56/9
Show the answer and explanation

Answer: C (72/25)

Explanation

a = 2r and a/(1 − r) = 8 give 2r = 8 − 8r, so r = 4/5 and a = 8/5. The first two terms add to 8/5 + 32/25 = 72/25.