Find the sum of the first 8 terms of the geometric progression (G.P.): 2, 6, 18, ...
A6560CORRECT
B3280
C6558
D6562
GCEFurther MathematicsSequences and Series2020
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Why the answer is A, and why the others tempt you.
The common ratio r = 6/2 = 3. The sum of n terms of a G.P. is S_n = a(r^n - 1)/(r - 1). So S_8 = 2(3^8 - 1)/(3 - 1) = 2(6561 - 1)/2 = 6560. Option C (6558) is a common arithmetic slip, and option B halves the answer incorrectly.
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