Question

[TRƯƠNG NGỌC NHÃ HÂN]

CMR 

\(1,\frac{a^2}{b^2}+\frac{b^2}{a^2}\ge\frac{a}{b}+\frac{b}{a}\)

\(2,Với a,b\ge1.CMR : a\sqrt{b-1}+b\sqrt{a-1}\le ab \)
\(3, a^2+b^2+c^2+d^2\ge\left(a+b\right)\left(c+d\right)\)