2.6.2.1 Trigonometric Fourier series

Problem: Expand function \( f(x)=\frac{x^{2}-1}{2} \) into a trigonometric Fourier series on the line segment \( [-\pi ; \pi] \). Plot the graphs of function \( f(x) \) and the sum of the Fourier series. Write down the first three partial sums of the Fourier series and approximately plot their graphs.