If-n-0-cosnsin-na-n-is-converge-then-a-is-1-a-Z-2-a-kpi-k-Z-3-a-R-2k-1-2-pi-k-Z-4-a-R-

Question Number 53619 by kaivan.ahmadi last updated on 23/Jan/19

If \underset{n = 0}{\sum^{\infty}} \frac{cosnsin (na)}{n} is converge then a is ?

1 . a \in Z

2 . a \in {k π : k \in Z}

3 . a \in R - {\frac{2 k + 1}{2} π : k \in Z}

4 . a \in R