Sum of a G.P. series is represented by the formula, Sn = \(a \frac{r^n - 1}{r-1}\), when r>1.
‘Sn’ represents the sum of the G.P. series up to nth terms, ‘a’ represents the first term, ‘r’ represents the common ratio and ‘n’ represents the number of terms.
Here,
a = √7
r = (ratio between the n term and n-1 term) √7 \(\div\) √21 = √3
n terms