$bài 1:cho: a,b,c>0$$a,t/m:a+b+c=3:CM:\frac{1}{2+a^2+b^2}+\frac{1}{2+b^2+c^2}\frac{1}{2+c^2+a^2}\leq \frac{3}{4}$$b,CM:\frac{\sqrt{ab}}{c+3\sqrt{ab}}+\frac{\sqrt{bc}}{a+3\sqrt{bc}}+\frac{\sqrt{ca}}{b+3\sqrt{ac}}\leq \frac{3}{4}$$c,CM:\frac{1}{(a+b)^2}+\frac{1}{(a+c)^2}\geq \frac{1}{a^2+bc}$$d,CM:\Sigma \frac{1}{a^5+b^2+c^2}\leq \frac{3}{a^2+b^2+c^2}$$e,CM:\Sigma \frac{a+b}{c^2+ab}\leq \frac{1}{b}+\frac{1}{a}+\frac{1}{c}$