6. (10%) The function f(x) be partially defined by $$f(x) = \begin{cases} \frac{\pi}{2}, & 0 < x \le \frac{\pi}{2}, \\ \pi - x, & \frac{\pi}{2} < x < \pi. \end{cases}$$ If f(x) is an odd periodic function with period $2\pi$, find its Fourier series representation.