Bài 1: Cho a,b,c>0 : abc=1. Cm:
$\frac{1}{a^{2}-a+1}+\frac{1}{b^{2}-b+1}+\frac{1}{c^{2}-c+1}\leq 3$
Bài 2: Cho $\left\{\begin{matrix} a,b,c,d\geq 0 & & & \\ a+b+c+d=4 & & & \\ a^{2}+b^{2}+c^{2}+d^{2}=7 & & & \end{matrix}\right.$
Cm: $a^{3}+b^{3}+c^{3}+d^{3}\leq 16$
Bài 3: Cho $\left\{\begin{matrix} a,b,c,d> 0 & & \\ a^{2}+b^{2}+c^{2}+d^{2}=1 & & \end{matrix}\right.$
Cm: $\frac{1}{1-ab}+\frac{1}{1-bc}+\frac{1}{1-cd}+\frac{1}{1-da}\leq \frac{16}{3}$
