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)

? a)

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

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

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

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