Chứng minh các bất đẳng thức sau:
1, $7<(1+\frac{1}{n})^{2n+1}\leqslant 8$
2, $C_{n}^{0}-xC_{n}^{1}+x^{2}C_{n}^{2}-x^{3}C_{n}^{3}+...+(-1)^{k}x^{k}C_(n)^{k}\geqslant 0. \forall x\epsilon [0,\frac{1}{n}],n\epsilon \mathbb{N^{*}}.$
3, $\frac{(a+b)^{n}-a^{n}-b^{n}}{2^{n}-2}\geqslant \sqrt{(ab)^{n}}$