# If z is one of the roots of unity with index n, find the sum

1. Apr 8, 2012

### tonit

1. The problem statement, all variables and given/known data
Given the fact that z is one of the n-th roots of unity, find the sum below:
1 + 2z + 3z2 + ... + nzn-1

2. Relevant equations

(1-x)(1+x+...+xn-1) = 1 - xn

3. The attempt at a solution
honestly I don't know how to do this. any help is appreciated

Last edited: Apr 8, 2012
2. Apr 8, 2012

### Eng1

..the hint for the solution is called complete induction. ;)

First of all you starting to show that the beginning of the sequence is true.
After that you show that its also true for n+1...

Try to make some sort of attempt to solve it...

3. Apr 8, 2012

### darkxponent

Divide eqn 2 with (1-x) and try solvin it using some calculus

4. Apr 8, 2012

### darkxponent

Yes u can use induction also. But try solving it using calculus. It is simpler and more intrestring

5. Apr 8, 2012

### tonit

what I'm trying to solve is this
1 + 2z + 3z2 + ... + nzn-1

6. Apr 8, 2012

### HallsofIvy

Staff Emeritus
Yes, you said that initially and you have two different suggestions as to how to do that. Have you tried either?

7. Apr 10, 2012

### tonit

I don't know how to apply induction to a sum. there is no "=" to prove. I have to find the sum, not prove something given. That's why I don't know how to apply induction.

8. Apr 10, 2012

### darkxponent

There is one relevant eqn missing

9. Apr 10, 2012

### I like Serena

I'm guessing that you're supposed to find a formula for the series (without 3 dots in it).

10. Apr 10, 2012

### morphism

Hint: What do you get if you differentiate x+x^2+...+x^n?