\(D=-\left|3x+1\right|+7-3x\)
\(D=7-3x-\left|3x+1\right|\)
\(D=7-3x-\left|-3x-1\right|\)
\(D\le\left|7-3x+3x+1\right|\)
\(D\le8\)
Dấu "=" xảy ra khi:
\(\left[{}\begin{matrix}\left\{{}\begin{matrix}3x+1\ge0\Rightarrow x\ge-\dfrac{1}{3}\\7-3x\ge0\Rightarrow x\le\dfrac{7}{3}\end{matrix}\right.\\\left\{{}\begin{matrix}3x+1< 0\Rightarrow x< -\dfrac{1}{3}\\7-3x< 0\Rightarrow x>-\dfrac{7}{3}\end{matrix}\right.\end{matrix}\right.\)
Vậy \(-\dfrac{1}{3}\le x\le\dfrac{7}{3}\)
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