2.6.2.19 Trigonometric Fourier series

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