\(B=\left|3x-1\right|+\left|3x-2\right|+\left|3x-3\right|+\left|3x-4\right|\)
\(=\left(\left|3x-1\right|+\left|4-3x\right|\right)+\left(\left|3x-2\right|+\left|3-3x\right|\right)\)
\(\ge\left|3x-1+4-3x\right|+\left|3x-2+3-3x\right|=3+1=4\)
Dấu \(=\) xảy ra khi \(\left\{{}\begin{matrix}\left(3x-1\right)\left(4-3x\right)\ge0\\\left(3x-2\right)\left(3-3x\right)\ge0\end{matrix}\right.\Leftrightarrow\dfrac{2}{3}\le x\le1\).
\(A=2-B\le2-4=-2\).
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