real analysis - Proof for $sum_{n=1}^{infty}frac{1}{n^2}=frac{pi^2}{6}$ without complexes?

This is what I needed. Practically, a link were also okay.

$$\sum_{n=1}^{\infty}\frac{1}{n^2}=\frac{\pi^2}{6}$$

Answer

Evaluating ζ(2) by Robin Chapman contains several proofs (~14 altogether). You can have a look through and find a nice one.