Why is $\lim_{n \to +\infty }{\sqrt[n]{a_1 a_2 \cdots a_n}} =\lim_{n \to +\infty}{a_n}$

Take the logarithm of both sides. Then you want to prove

$$\lim_{n\rightarrow \infty} \frac{1}{n}\sum_1^n a_i = \lim_{n\rightarrow \infty} a_n.$$

This is a standard result about Cesàro means.

Take the log of the $n$-root, and applied the Cesaro theorem to it, showing that it will converge to the log of $(a_n)_n$'s limit. Take the exp to finish.