$$\begin{cases}x+y+z=1 \\ x^{2}+y^{2}+x^{2}=1 \end{cases}$$ và $$x^{3}+y^{3}+x^{3}=1$$
Áp dụng hằng đẳng thức:$$x^{3}+y^{3}+z^{3}-3xyz=(x+y+x)(x^{2}+y^{2}+z^{2}-xy-yz-zx)\Leftrightarrow 1-3xyz=1(1-xy-yz-zx)\Leftrightarrow 3xyz=xy+yz+zx$$
Ta có:$1=(x+y+x)^{2}=x^{2}+y^{2}+z^{2}+2xy+2yz+2zx\Leftrightarrow 1=1+2(xy+yz+zx)\Leftrightarrow xy+yz+zx=0\Rightarrow 3xyx=0$
$\Leftrightarrow x=0$ hoặc y=0 hoặc z=0
+)Nếu x=0$\Rightarrow \begin{cases}y+z=1 \\ y^{2}+z^{2}= 1\end{cases}$ và $y^{3}+z^{3}=1$
Do đó suy ra đc:$y^{2}+2yz+z^{2}=1\Rightarrow 2yz=0$
suy ra:y=0;z=1 hoặc z=0;y=0
Vậy ta có (x;y;z)=(0;0;1) hoặc (x;y;z)=(0;1;0)

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