A function is odd if it satisfies \( f(-x) = -f(x) \). For odd functions, the integral over a symmetric interval \( [-a, a] \) equals zero:\[\int_{-a}^a f(x) \, dx = 0.\]This occurs because the positive and negative areas under the curve cancel each other out.
Final Answer: \( \boxed{f(-x) = -f(x)} \)