toán nâng cao lớp 7

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khaiproqn81

$x^2+y^2+z^2=xy+yz+xz \\\Longleftrightarrow 2x^2+2y^2+2z^2=2xy+2yz+2xz \\ \Longleftrightarrow x^2-2xy+y^2+y^2-2yz+z^2+z^2-2zx+x^2=0 \\ \Longleftrightarrow (x-y)^2+(y-z)^2+(x-z)^2=0 \\ \Longleftrightarrow x=y=z$
 
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