Arithmetic Sum on n terms $$ \begin{align*} S_n &= \frac{n}{2}[2a + (n-1)d] \\ S_n &= \frac{n}{2}(a + l) \end{align*} $$ Geometric Sum on n terms $$ \begin{align*} S = \frac{a(1 - r^n)}{1-r} \end{align*} $$