Let $f$ be a $2π$ -periodic square wave function so that
$$f\, = -1 \quad -π \le x<0$$
$$f=1 \qquad 0 \le x< π$$
$S_{2n-1}(x)$ is the $(2n-1)st$ Fourier polynomial of $f$. Prove that it can be written as:
$$S_{2n-1}(x)=\frac{1}{nπ}\int_0^{2nx} \frac{\sin t } {\sin {\frac {t} {2n}}...