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In a GP 2, 6, 18, 54 .... find how many terms are there, if their sum is 728.

A) 5
B) 6
C) 12
D) 7

Correct Answer:


B) 6

Description for Correct answer:
Given GP is 2, 6, 18, 54

\( \Large S_{n} \) = 728 Here a = 2, r = 3

\( \Large S_{n} \) = \( \Large \frac{a \left(r^{n}-1\right) }{r-1} \)

728 = \( \Large \frac{2 \left(3^{n}-1\right) }{3-1} = 3^{n}-1 \)

\( \Large 3^{n} = 728+1 = 729 \)

\( \Large 3^{n} = 3^{6} \)

n = 6

Part of solved Factorisation questions and answers : >> Elementary Mathematics >> Factorisation

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