Hello, afeasfaerw23231233!
This is a geometric series . . .
. . first term: \cos\theta + i\sin\theta)" alt="a \:=\\cos\theta + i\sin\theta)" />
. . common ratio: \cos\theta + i\sin\theta)" alt="r \:=\\cos\theta + i\sin\theta)" />
. . number of terms:
Recall that the sum of a geometric series is: .
So we have: .
Got it?