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JAMB 2004 · UME · Q19

Given that the first and fourth terms of a G.P. are 6 and 162 respectively, find the sum of the first three terms of the progression.

Worked solution (try it first)
  1. The 4th term of a G.P. is ar3ar^3, so 6r3=1626r^3 = 162 and r3=27r^3 = 27.
  2. Take the cube root: r=3r = 3.
  3. The first three terms are 6, 18 and 54, and their sum is 78, option D.

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