Cho
$$f(a,b,c,d)=\frac{a}{b+c+d}+\frac{b}{c+d+a}+\frac{c}{d+a+b}+\frac{d}{a+b+c}$$
CMR nếu
$$\frac{1}{2}\geq a,b,c,d\geq 0$$
thì
$$f(a,b,c,d)\geq f(1-a,1-b,1-c,1-d)$$
Nếu
$$a,b,c,d \in \left[\frac{1}{2},1\right]$$
thì
$$f(a,b,c,d)\le f(1-a,1-b,1-c,1-d)$$
