Question

Find the sum of G.P. :

1 + 3 + 9 + 27 + .......... to 12 terms. 

Answer 1

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