cho x,y,z lớn hơn hoặc bằng 1

a)\(\frac{1}{1+x^2}+\frac{1}{1+y^2}\ge\frac{2}{1+xy}\)

b)\(\frac{1}{1+x^2}+\frac{1}{1+y^2}+\frac{1}{1+z^2}\ge\frac{3}{1+xyz}\)

Cho a,b,c >0 , chứng minh rằng

a) \(\frac{a^3}{b}\ge a^2+ab-b^2\)

b)\(\frac{a^3}{b}+\frac{b^3}{c}+\frac{c^3}{a}\ge ab+bc+ca\)