1.

How many terms of G.P. `3,3^2,3^3,dotdotdot`are needed to give the sum 120?

Answer» In the given GP,
`a = 3, r = 3`
`S_n = 120`
`:. a(r^n-1)/(r-1) = 120`
`=>3(r^n-1)/(3-1) = 120`
`=>3^n -1 = 80 => 3^n = 81`
`=> 3^n = 3^4`
`n = 4`


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