Madrid 2014-P2

Calcular
a) \(\displaystyle\prod_{k=1}^{n -1} \left ( e^{2k\pi {\rm i} /n} -1 \right ).\qquad\qquad\) b) \(\displaystyle\require{AMSmath}\DeclareMathOperator{\sin}{sen}\prod_{k=1}^{n -1} \sin \frac{k\pi}{n}\).