$$f(t)=a_0+\sum _{n=1}^{\mathrm{\infty }}[a_n\cos (nt)+b_n\sin (nt)]$$ $$a_0=\frac{1}{2L}{\int }_{-L}^{L}f(t)dt$$ $$a_n=\frac{1}{L}{\int }_{-L}^{L}f(t)\cos \frac{n\pi t}{L}dt$$ $$b_n=\frac{1}{L}{\int }_{-L}^{L}f(t)\sin \frac{n\pi t}{L}dt$$

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