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Let `alpha and beta` be the roots of `x^(2)+x+1=0`. If n be positive integer, then `alpha^(n)+beta^(n)` is

A

`2cos""(2npi)/(3)`

B

`2sin""(2npi)/(3)`

C

`2cos""(npi)/(3)`

D

`2sin""(npi)/(3)`

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