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JAMB Mathematics 1990 Question 28

Question 28

Progressions

Find the sum of the first 18 terms of the progression 3, 6, 12,...

  1. A.3(217 − 1)
  2. B.3(218) − 1
  3. C.3(218 + 1)
  4. D.3(218 − 1)
Show the answer and explanation

Answer: D (3(218 − 1))

Explanation

This is a G.P. with a = 3 and r = 2, so S18 = 3(218 − 1)/(2 − 1) = 3(218 − 1).