How to prove that $\int_{0}^{\pi}\left|{\sin{nx}\over x}\right|\,\mathrm dx\ge{2\over\pi} \sum\limits_{k = 1}^n \frac1k$
Prove $\forall n \in \mathbb{N}: \int_{0}^{\frac{\pi}{2}} |\frac{\sin(nx)}{x}|dx \geq \frac{2}{\pi}\sum_{k=1}^{n}\frac{1}{k}$