a, cho a,b,c $\in \begin{bmatrix} 0 &;1 \end{bmatrix}$
cmr $\frac{a}{1+bc}+\frac{b}{1+ac}+\frac{c}{1+ab}\leq 2$
b, cho $\frac{a}{b}< \frac{c}{d}.$
cmr $\frac{a}{b}< \frac{ab+cd}{b^{2}+d^{2}}< \frac{c}{d}$
và $\frac{a}{b}< \frac{a+c}{b+d}< \frac{c}{d}$ nếu $b,d> 0$