1) Cho a,b,c,d>0 thỏa:
$\frac{1}{2+a^2}+\frac{1}{2+b^2}+\frac{1}{2+c^2}+\frac{1}{2+d^2}=\frac{1}{2}$
CMR: $abcd\geq ab+ac+ad+bc+bd+cd$;
2) Cho a,b,c,d>0 thỏa:
$\frac{1}{1+a^4}+\frac{1}{1+b^4}+\frac{1}{1+c^4}+\frac{1}{1+d^4}=1$
CMR: $(\frac{1}{a}+\frac{1}{c})(\frac{1}{b}+\frac{1}{d})\leq\frac{4}{\sqrt3}$
