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\(x^2-81=0\)
\(\Rightarrow\left(x+9\right)\left(x-9\right)=0\)
\(\Rightarrow\orbr{\begin{cases}x+9=0\\x-9=0\end{cases}\Rightarrow\orbr{\begin{cases}x=-9\\x=9\end{cases}}}\)
vậy...
\(6x-x^2-9=0\)
\(\Rightarrow-\left(x^2-6x+9\right)=0\)
\(\Rightarrow\left(x-3\right)^2=0\)
\(\Rightarrow x=3\)
\(x^2-81=0\)
\(x^2=81\)
\(\Leftrightarrow x=\pm9\)
\(6x-x^2-9=0\)
\(-x^2+6x-9=0\)
\(-\left(x^2-6x+9\right)=0\)
\(x^2-6x+9=0\)
\(\left(x-3\right)^2=0\)
\(\Rightarrow x-3=0\Leftrightarrow x=3\)
