\(-\text{Theo bài ra: }D=\dfrac{x}{2x+y+z}+\dfrac{y}{2y+z+x}+\dfrac{z}{2z+x+y}\)
\(-\text{Đặt }\left\{{}\begin{matrix}a=2x+y+z\\b=2y+z+x\\c=2z+x+y\end{matrix}\right.\Rightarrow a+b+c=4\left(x+y+z\right)\)
\(\Rightarrow a-\dfrac{a+b+c}{4}=x\)
\(\Rightarrow x=\dfrac{3a-b-c}{4}\)
\(-\text{Tương tự: }\left\{{}\begin{matrix}y=\dfrac{3b-c-a}{4}\\z=\dfrac{3c-a-b}{4}\end{matrix}\right.\)
Suy ra \(D=\dfrac{3a-b-c}{4a}+\dfrac{3b-3c-a}{4b}+\dfrac{3c-a-b}{4c}\)
\(D=\dfrac{9}{4}-\left(\dfrac{b}{4a}+\dfrac{c}{4a}+\dfrac{c}{4b}+\dfrac{a}{4b}+\dfrac{a}{4c}+\dfrac{b}{4c}\right)\)
\(D=\dfrac{9}{4}-\dfrac{1}{4}\left[\left(\dfrac{b}{a}+\dfrac{a}{b}\right)+\left(\dfrac{c}{a}+\dfrac{a}{c}\right)+\left(\dfrac{c}{b}+\dfrac{b}{c}\right)\right]\)
- Theo bất đẳng thức Cosi, ta có: \(\left\{{}\begin{matrix}\dfrac{b}{a}+\dfrac{a}{b}\ge2\\\dfrac{c}{a}+\dfrac{a}{b}\ge2\\\dfrac{c}{b}+\dfrac{b}{c}\ge2\end{matrix}\right.\)
Suy ra \( D\le\dfrac{9}{4}-\dfrac{1}{4}.6=\dfrac{9}{4}-\dfrac{6}{4}=\dfrac{3}{4}\)
Vậy \(D\le\dfrac{3}{4}\left(đpcm\right)\)
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