Mathematics

Problem

Which of the following gives the sum

S_n

of the first

n

terms of a geometric progression whose first term is

a

and common ratio

r\,(r\neq1)

, when the denominator is expressed as

(r-1)

? a)

a\dfrac{1-r^{n}}{r-1}

a\dfrac{r^{n-1}-1}{r-1}

a\dfrac{r^{n}-1}{r-1}

a\dfrac{r^{n}+1}{r-1}