Mathematics

Problem

Which of the following correctly gives the sum

S_n

of the first

n

terms of a geometric progression with first term

a

and common ratio

r

where

r \neq 1

? a)

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

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

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

S_n = \dfrac{a}{1-r}