a: =>|x-3|=12-5x-8=4-5x
\(\Leftrightarrow\left\{{}\begin{matrix}x< =\dfrac{4}{5}\\\left(4-5x\right)^2=\left(x-3\right)^2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x< =\dfrac{4}{5}\\\left(5x-4-x+3\right)\left(5x-4+x-3\right)=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x< =\dfrac{4}{5}\\\left(4x-1\right)\left(6x-7\right)=0\end{matrix}\right.\Leftrightarrow x=\dfrac{1}{4}\)
b: |2-3x|=|6-x|
=>|3x-2|=|x-6|
=>3x-2=x-6 hoặc 3x-2=6-x
=>2x=4 hoặc 4x=8
=>x=2
c: |-21|-|5-x|=16
=>|x-5|=5
=>x-5=5 hoặc x-5=-5
=>x=10 hoặc x=0
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