For the geometric series $a + ar + ar^2 + \ldots$ with $r \ne 1$, the partial sum up to $n$-th term is —

• a$a \cdot \dfrac{1 - r^n}{1 - r}$
• b$a \cdot \dfrac{1 + r^n}{1 + r}$
• c$a \cdot \dfrac{r^n - 1}{r}$
• d$\dfrac{a}{1 - r}$