I am having difficulty solving this math problem:

Suppose it is known that |x| < 1. What then is the sum from n=1 to n=infinity of nx^n?

I know how to do it if there is a constant in front of the x, but I don't know what to do since there's a variable in front of it.

Anybody have any ideas, it's really starting to annoy me?

Thanks a Lot

-Nertil