79) Cho $a;b;c>0$ và $abc=1$. Cmr: $\sum \frac{1}{a^3(b+c)}\geq \frac{3}{2}$
80) Cho $a;b;c;p;q>0$. Cmr: $\sum \frac{a}{pb+qc}\geq \frac{3}{p+q}$
81) Cho $x;y;z>0$. Cmr: $\sum \frac{2}{x+y}\geq \frac{9}{x+y+z}$
82) Cho $a;b;c>0$. Cmr: $\sum \frac{a^2+b^2}{a+b}\geq a+b+c$
83) Cho $x;y;z>0$. Cmr: $\sum \frac{x}{x+2y+3z}\geq \frac{1}{2}$
84) Cho $\left\{\begin{matrix}a;b;c>0 & & \\ 3(ab+bc+ca)=1 & & \end{matrix}\right.$. Cmr: $\sum \frac{a}{a^2-bc+1}\geq \frac{1}{a+b+c}$
81 $\sum \frac{2}{x+y}\geq 2\frac{9}{2(x+y+z)}= \frac{9}{x+y+z}$(bđt bcs)
dấu = xảy ra x=y=z