Đặt \(A=\left|x+5\right|+\left|x+1\right|+4=\left|x+5\right|+\left|-x-1\right|+4\)
\(\Rightarrow A\ge\left|x+5-x-1\right|+4=4+4=8\\ A_{min}=8\Leftrightarrow-\left(x+5\right)\left(x+1\right)\ge0\Leftrightarrow\left(x+5\right)\left(x+1\right)\le0\Leftrightarrow-5\le x\le-1\)
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