# Sum of series

1. Jul 27, 2008

### Lance WIlliam

1. The problem statement, all variables and given/known data

$$\Sigma$$ (1/2^n+1/3^n)
n=1
2. Relevant equations

3. The attempt at a solution
I get 7/2
by using a/1-r to both than adding....what am I doing wrong

2+3/2= 7/2

2. Jul 27, 2008

### Defennder

I didn't get 7/2. Your technique seems ok. Must be some calculation error.

3. Jul 27, 2008

### Dick

You are using a/(1-r) with a=1. a is supposed to be the first term in the series. The sum starts at n=1.

4. Jul 27, 2008

### HallsofIvy

Staff Emeritus
The standard geometric series starts at n=0. These are not geometric series because they are missing the "n=0" term. Of course, for each of the two series, that would just be 1/20= 1 and 1/30= 1. Since those terms are missing, you need to subtract 2 from your result.