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2) Let $f$ be an odd piecewise continuous function of period $4L$ and which is even with respect $x=L.$ Show that the Fourier series of $f$ is $\displaystyle\sum_{n\ge1}b_{2n-1}\sin\frac{(2n-1)\pi}{2L}x,$ where $b_{2n-1}=\displaystyle\frac2L\int_0^L f(x)\sin\frac{(2n-1)\pi}{2L}x\,dx.$

__Attempts__:

1) I don't know how to find the Fourier series here, how to work with a non-symmetric interval?

2) No ideas here, how to start?