#### Answer

$\dfrac{a_1(1-r^n)}{1-r}$;
$\underline{\text{first term}}$;
$\underline{\text{common ratio}}$

#### Work Step by Step

RECALL:
The sum of the first $n$ terms, $S_n$, of a geometric sequence can be found using the formula
$S_n=\dfrac{a_1(1-r^n)}{1-r}$
where
r = common ratio
$a_1$ = first term
Thus, the missing expressions in the given statements are $\dfrac{a_1(1-r^n)}{1-r}$, $\underline{\text{first term}}$, and $\underline{\text{common ratio}}$, respectively.