数列

等差数列

$a_n = a_1 + (n-1)d$

$S_n = \frac{n}{2} (a_1 + a_n) = na_1 + \frac{n(n-1)}{2}d$

等比数列

$a_n = a_1 \cdot q^{(n-1)}$

$S_n = \frac{a_1(1-q^n)}{1-q}, q \neq 1$