Mathematics

Problem

Which of the following expressions represents the sum

S_n

of the first

n

terms of a geometric progression whose first term is

a

and common ratio is

r\,(r\neq1)

? a)

S_n=\dfrac{a(r^{n}-1)}{r}

S_n=\dfrac{a(1-r^{n})}{1-r}

S_n=a\,r^{n-1}

S_n=\dfrac{a(r^{n}-1)}{1-r}