\(x^3=3y^2-3y+1=3\left(y-\dfrac{1}{2}\right)^2+\dfrac{1}{4}\ge\dfrac{1}{4}\)
\(\Rightarrow x\ge\dfrac{1}{\sqrt[3]{4}}>\dfrac{1}{2}\)
Tương tự ta có \(y;z>\dfrac{1}{2}\)
\(\Rightarrow x+y-1>0;y+z-1>0;z+x-1>0\)
TH1: \(x\ge y\Rightarrow x^3\ge y^3\Rightarrow3y^2-3y+1\ge3z^2-3z+1\)
\(\Rightarrow y^2-z^2-y+z\ge0\Rightarrow\left(y-z\right)\left(y+z+1\right)\ge0\)
\(\Rightarrow y-z\ge0\Rightarrow y\ge z\Rightarrow x\ge z\) (1)
Cũng do \(y\ge z\Rightarrow y^3\ge z^3\)
\(\Rightarrow3z^2-3z+1\ge3x^2-3x+1\Rightarrow z^2-x^2-z+x\ge0\)
\(\Rightarrow\left(z-x\right)\left(z+x+1\right)\ge0\Rightarrow z\ge x\) (2)
Từ (1);(2) \(\Rightarrow x=y=z\)
TH2: \(x\le y\), hoàn toàn tương tự ta cũng chứng minh được \(x=y=z\)
Thay vào hệ ban đầu:
\(\left\{{}\begin{matrix}x^3-3x^2+3x=1\\y^3-3y^2+3y=1\\z^3-3z^2+3z=1\end{matrix}\right.\) \(\Rightarrow x=y=z=1\)
