URGENT: Why does this tern = 0??

[Baggio]

Can anyone tell me why the term in the bracket = 0??

where i= root -1 and all vars are integers

I'm at a loss :-/

[Tide]

Each exponential in the sum represents a point in the complex plane and if you look at the terms carefully you will see that the set of points comprise the vertices of a regular polygon. Think of each point as being a vector from the origin to the given point. Being the vertices of a regular polygon, the vectors must add to zero! That is, unless c is a multiple of q/r in which case all the vertices converge to a single point.

[Baggio]

Hehe, ok I think that makes sense.. thanks