4 Sean $a_1, a_2, \cdots, a_n$ números reales. Demuestre que: $\sum_{k=1}^{n+1} (a_k-a_{k-1})^2 \le 2(1+ \cos \frac{\pi}{n+1} )\sum_{k=1}^{n} a_k^2$ , donde $a_0 = a_{n+1} = 0$ .