Cho 3 số thực dương a,b,c. Chứng minh:
1.
$(\dfrac{a}{b}+\dfrac{b}{c}+\dfrac{c}{a})^2 \ge (a+b+c)(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c})$
2.
$(a^2+1)(b^2+1)(c^2+1) \ge \dfrac{5}{16}(a+b+c+1)^2$
3.
$\dfrac{a^3b}{1+ab^2}+\dfrac{b^3c}{1+bc^2}+\dfrac{c^3a}{1+ca^2} \ge \dfrac{abc(a+b+c)}{1+abc}$