If the second and fourth term of a G.P are 8 and 32 respectively,what is the sum of the first four terms?

  • A 28
  • B 40
  • C 48
  • D 60
  • E 68
waec 1989mathematics

The correct answer is D. 60

ar = 8 .....(1)

ar3 = 32 .....(2)

divide (2) by (1)

r2 = 4 r = 2 ∴ a = 4

S = a(rn - 1)/r - 1

S = 4(24 - 1)/2 - 1 = 60

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