|3x|=x+7
\(\Leftrightarrow\left\{{}\begin{matrix}\left(3x\right)^2-\left(x+7\right)^2=0\\x>=-7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\left(2x-7\right)\left(4x+7\right)=0\\x>=-7\end{matrix}\right.\Leftrightarrow x\in\left\{\dfrac{7}{2};-\dfrac{7}{4}\right\}\)
\(3x\ge0\Leftrightarrow x\ge0\)
\(3x=x+7\)
\(\Leftrightarrow2x=7\)
\(\Leftrightarrow x=\dfrac{7}{2}\)
|3x|=x+7
=>3x=x+7 hoặc -3x=x+7
=>3x-x-7=0 -3x-x-7=0
=>2x-7=0 -4x-7=0
=>x=7/2 x=-7/4
