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Question
Find the sum of G.P. :
1 + 3 + 9 + 27 + .......... to 12 terms.
Solution
Given G.P. : 1 + 3 + 9 + 27 + ..........
Here,
First term, a = 1
Common ratio, r =`3/1` = 3 ...(∵ r > 1)
Number of terms to be added, n = 12
∴ `S_n = (a(r^n - 1))/(r - 1)`
`=> S_12 = (1(3^12 - 1))/(3 - 1)`
= `(3^12 - 1)/2`
= `(531441 - 1)/2`
= `531440/2`
= 265720
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