Marinada '25

[lang=hr] Odredi $$\sum_{n = 1}^{\infty} \frac{1}{n^2 (n + 1)^2}.$$ [/lang] [lang=en] Calculate $$\sum_{n = 1}^{\infty} \frac{1}{n^2 (n + 1)^2}.$$ [/lang]