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Question
Find the sum of first six terms of the G.P. 5, 15, 45, …
Solution
G.P. 5, 15, 45
n = 6, a = 5, r = `15/5` = 3 > 1
∴ Sn = `"a"(("r"^"n" - 1))/("r" - 1)`
S6 = `5((3^6 - 1)/(3 - 1))`
= `5((3^6 - 1))/2`
= `5/2(729 - 1)`
= `5/2 xx 728`
= 5 × 364
= 1820
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