\[sina + sin(a+b)+sin(a+2b)+....+sin(a+(n-1)b)= \frac{\sin (a+(n-1)\frac{b}{2}))\sin \frac{nb}{2}}{sin\frac{b}2{}} \]
Boundary condition - Denominator should not be equal to zero
\[b\neq 0 \]
For this to be true b should not be equal to 0. because If it happens then sin0=0 . Hence fraction will not be defined
