## Week 2-Geometric Series

How to find $S_{n}$

If you are given with the geoettic series of

80 – 40 + 20 – …               and it asks to find $S_{6}$

Find the r

Which is -2

Then put all the numbers in the equation:

$S_{6}=\frac{a(r^8 -1)}{r-1}$

Let’s go:

$S_{6}=\frac{80(-2^6 -1)}{-2-1}$ $S_{6}=\frac{80(63)}{-3}$ $S_{6}=\frac{5040}{-3}$ $S_{6}=-1680$