Mình xin chém câu bất (ms nghĩ ra @@)
Ta có: VT = $\frac{1}{1+\frac{b^{2}}{a}}+\frac{1}{1+\frac{c^{2}}{b}}+\frac{1}{1+\frac{a^{2}}{c}}$
Mà $(1+\frac{b^{2}}{a})(1+a)\geq (1+b)^{2}$ => VT $\leq \frac{1+a}{(1+b)^{2}}+\frac{1+b}{(1+c)^{2}}+\frac{1+c}{(1+a)^{2}}$
$=\sum \frac{1+a}{(a+2b+c)^{2}}\leq \frac{1}{4}\sum \frac{1+a}{(a+b)(b+c)}=\frac{1}{4}\frac{\sum (1+a)(1-b)}{\prod(1-a)}=\frac{1}{4}*\frac{3-(ab+bc+ca)}{ab+bc+ca-abc}$
Ta chứng minh $\frac{3-(ab+bc+ca)}{ab+bc+ca-abc}\leq \frac{1}{a}+\frac{1}{b}+\frac{1}{c}$
$<=>(ab+bc+ca)^{2}\geq 3abc$ (đúng do a+b+c = 1)
Dấu "=" xảy ra khi a = b = c = $\frac{1}{3}$.