Với mọi x ta có :
\(\left\{{}\begin{matrix}\left|x+3\right|\ge0\\\left|x+1\right|\ge0\end{matrix}\right.\)
\(\Leftrightarrow\left|x+3\right|+\left|x+1\right|\ge0\)
\(\Leftrightarrow3x\ge0\)
\(\Leftrightarrow x\ge0\)
Với \(x\ge0\) ta dc :
+) \(\left|x+3\right|=x+3\)
+) \(\left|x+1\right|=x+1\)
\(\Leftrightarrow\left|x+3\right|+\left|x+1\right|=x+1+x+3\)
\(\Leftrightarrow x+1+x+3=3x\)
\(\Leftrightarrow2x+4=3x\)
\(\Leftrightarrow x=4\)
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