While deriving complex fourier series I have reached at $\displaystyle c_0+\sum_{n=1}^{n=\infty}c_ne^{\frac{in\pi x}{l}}+\sum_{n=-1}^{n=-\infty}c_ne^\frac{in\pi x}{l}$.

Can anyone tell me what combined expression (pattern) can be written for this and why?