1)cho a,b,c>0, a+b+c=3. CMR: $\sqrt{a}+\sqrt{b}+\sqrt{c} \geqslant ab +bc +ca$
2)cho a,b,c>0. CMR: $\frac{a^{2}}{b+c}+\frac{b^{2}}{c+a}+\frac{16c^{2}}{a+b}\geqslant \frac{1}{9}(64c-a-b)$
3) cho a,b,c$\geq$0. a+b+c=3. CMR: $a^{2}+b^{3}+c^{3}+ab+bc+ca\geq 6$