giả sử $a,b,c\epsilon R^{*}$ cho abc =1.
chứng minh rằng
1)$\frac{1}{\sqrt{a^3+2b^3+6}}+\frac{1}{\sqrt{b^3+2c^3+6}}+\frac{1}{\sqrt{c^3+2a^3+6}}\leq 1$
2)$\frac{ab}{2b+c}+\frac{bc}{2c+a}+\frac{ca}{2a+b}\geq 1$
3)$\frac{a}{(ab+a+1)^2}+\frac{b}{(bc+b+1)^2}+\frac{c}{(ca+c+1)^2}\geq \frac{1}{a+b+c}$
thank nhiều