Question

Find the sum upto n terms of the G.P. √7,√21,3√7,.......

Answer 1

Hello Keesan
The given GP is √7,√21,3√7,.......
the common ratio of the GP is(√21/√7)=√3
we know that the sum of first n terms of the GP with first term a and common ratio r is
$s=a(r^{n}-1)/(r-1)$
so the sum of GP is 
$\sqrt{7}( \sqrt{3}^{n}-1)}/( \sqrt{3}-1)$
Hope it helps.

Answer 2

a=√7 r= √21/√7 = √3 >1

Sn = a( rn - 1) / r-1

= √7 [ (√3)n - 1]/ √3-1

= √7 ( √3+1) [(√3)n - 1 / 2