# Homework Help: What does 2n on top of the summation expression do diferently than just n?

1. Sep 20, 2012

### student34

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

2n
Ʃ (k)
k=1

3. The attempt at a solution

2n
Ʃ (k) = 2n(2n+1)/2 (This is just a shot in the dark.)
k=1

2. Sep 20, 2012

### Dick

Sure. If you've shown the sum from 1 to n of k is n*(n+1)/2, then the sum to 2n is just 2n*(2n+1)/2. It's just substitution.

3. Sep 20, 2012

### student34

So would this work too?

2n+1
Ʃ (k) = (2n+1)((2n+1)+1)/2
k=1

4. Sep 20, 2012

### Dick

Sure. Same thing.

5. Sep 21, 2012

### Ray Vickson

$$\sum_{k=1}^N k = \frac{N(N+1)}{2},$$
so if you put N = 2n you get the stated result. If you set N = 2n+1 you get the other result you stated.

RGV