$1, a,b,c > 0; ab+bc+ca=abc$. Tìm $max$:
$\frac{a}{b(a+1)}+\frac{b}{ac(b+1)}+\frac{c}{ab(c+1)}$
$2, a,b,c >0; a^2+b^2+c^2=4\sqrt{abc}.$ Chứng minh $a+b+c>2\sqrt{abc}$
$3, x>1; y >0$. Chứng minh $\frac{1}{(x-1)^3}+(\frac{x-1}{y})^3+\frac{1}{y^3} \ge 3(\frac{3-2x}{x-1}+\frac{x}{y})$