1. Cho $a_1,a_2,...a_{n} \geq 0$ ; $b_1,b_2...,b_{n} \geq 0$. Chứng minh:
$$\sqrt[n]{a_1.a_2...a_{n}}+\sqrt[n]{b_1.b_2...b_{n}} \leq \sqrt[n]{(a_1+b_1)....(a_{n}+b_{n})}$$
2. Cho $a,b,c>0$. Chứng minh
$$\sum \dfrac{a^2}{a+b} \geq \dfrac{\sqrt{2}}{4}.(\sum \sqrt{a^2+b^2})$$