# Week 2 Pre-calculus 11

Finite Geometric Series : you use this formula $S_{n}=\frac{a(r^n -1)}{r-1}$

Geometric Series : {

Infinite Geometric series : Because Diverging r>1 or r<-1，so no sum

For example : t=-0.5, r=-3

So -0.5,1.5,-4.5,13.5. It gets bigger and it gets smaller, no fixed size,either keep getting bigger or you keep getting smaller.

Converging: you can use this formula -1<r<1  $S_{n}$=a/1-r  n= ∞