# Week Two: Geometric Series

Geometric Series: 3+9+18+36…

You know its a geometric series because it is being multiplied every time by 3.  This is called the “r” or the ratio. If you don’t know what the ratio is by looking at the series then we take a term and the one before it and divide it. (9/3)

Using the geometric series above find what $S_26$ would be.

Find R: 9/3= 3

Use formula: $S_26$ = $\frac{-5(r^n-1)}{r-1}$

$S_26$ = $\frac{-5(3^{26}-1)}{3-1}$

$S_26$ = -6,354,664,570,820