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tylerc1991
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Homework Statement
Show that if c is any nth root of unity other than unity itself that:
1 + c + c^2 + ... + c^(n-1) = 0
Homework Equations
1 + z + z^2 + ... + z^n = (1 - z^(n+1)) / (1 - z)
The Attempt at a Solution
c is an nth root of unity other than unity itself => (1-c) =/= 0.
so,
1 + c + c^2 + ... + c^(n-1) = (1 - c^n) / (1 - c) (= 0 by assumption)
hence,
(1 - c)(1 + c + c^2 + ... + c^(n-1)) = 0
so either (1 - c) = 0 or (1 + c + c^2 + ... + c^(n-1)) = 0
but (1 - c) =/= 0 by definition
so (1 + c + c^2 + ... + c^(n-1)) = 0