Cho $a,b,c\geq 0$, chứng minh $\sqrt{\frac{a+2b}{3}}+\sqrt{\frac{b+2c}{3}}+\sqrt{\frac{c+2a}{3}}\geq \sqrt{a}+\sqrt{b}+\sqrt{c}$
-Ta có: \[\frac{{a + b + b}}{3} \ge \frac{{{{(\sqrt a + \sqrt b + \sqrt b )}^2}}}{9} = > \sqrt {\frac{{a + 2b}}{3}} \ge \frac{{\sqrt a + 2\sqrt b }}{3}\].
-Tương tự, ta có: \[\sqrt {\frac{{b + 2c}}{3}} \ge \frac{{\sqrt b + 2\sqrt c }}{3};\sqrt {\frac{{c + 2a}}{3}} \ge \frac{{\sqrt c + 2\sqrt a }}{3}\].
-Cộng vế với vế suy ra đpcm.