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If `omega` is a complex nth root of unity, then `sum_(r=1)^n(a+b)omega^(r-1)` is equal to `(n(n+1)a)/2` b. `(n b)/(1+n)` c. `(n a)/(omega-1)` d. none of these

A

`(n(n+1)a)/(2 omega)`

B

`(nb)/(1-n)`

C

`(na)/(omega-1)`

D

None of these

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