5. (15%) Let \( f(x) \) be an odd periodic function with period \( 2\pi \). \( f(x) = 0 \) for \( 0 < x < \pi / 2 \) and \( f(x) = x \) for \( \pi / 2 < x < \pi \). Find the Fourier series representation of \( f(x) \).