Hoang Tung 1998

Cho $a,b,c> 0.$. CMR:$\frac{1}{a^{2}+bc+ac}+\frac{1}{b^{2}+ca+ab}+\frac{1}{c^{2}+ab+bc}\leq \frac{(a+b+c)^2}{(ab+bc+ac)^2}$

MOD : Học đặt tiêu đề

Cho $a,b,c\geq 0.$. CMR:

 A=$\sum \frac{a^{2}}{b^{2}-bc+c^{2}}\geq a+b+c$

Cho a,b,c >0.CM : $\left ( a+b \right )^2(b+c)^2(c+a)^2\geq abc.(a+b+2c)(b+c+2a)(c+a+2b)$