\(x^3+3x^2+5x+3=\left(x^2+3\right)\sqrt{x^2+1}\) ( ĐK : \(x\in R\))
\(\Leftrightarrow\left(x+1\right)^3+2\left(x+1\right)=\sqrt{\left(x^2+1\right)^3}+2\sqrt{x^2+1}\)
Xét hàn số : \(f\left(t\right)=t^3+2t\)
\(f'\left(t\right)=3t^2+2>0\)
\(\Rightarrow x+1=\sqrt{x^2+1}\)
\(\Leftrightarrow x=0\)
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