Ira S. answered 09/26/14
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I just did the cube roots of 1 and the fourth roots of 1 and it looks like your answer is going to be that the sum is
n*x^(n-1) / (x^n - 1)
For the cube roots it came out to be 3x^2 / (x^3 - 1)
For the fourth roots it came out to be 4x^3 / (x^4 - 1)
Just because this looks like the general pattern, doesn't mean it's right. Working on a proof for this.