Cho a,b,c dương Chứng minh
$a,2(a^{2}+1)(b^{2}+1)(c^{2}+1) \geq (a+1)(b+1)(c+1)(abc+1)$
$b,8(a^{2}+1)^{3}(b^{2}+1)^{3}(c^{2}+1)^{3} \geq (a+1)^{3}(b+1)^{3}(c+1)^{3}(a^{3}+1)(b^{3}+1)(c^{3}+1)$
$b,\frac{a^{3}+abc}{b+c}+\frac{b^{3}+abc}{c+a}+\frac{c^{3}+abc}{a+b} \geq a^{2}+b^{2}+c^{2}$