\(x+y=-1\Rightarrow y=-1-x\)
Thế vào pt đầu:
\(x^3-\left(-1-x\right)^3=3\left(x+1+x\right)\)
\(\Leftrightarrow\left(2x+1\right)\left(x^2+x+1\right)=3\left(2x+1\right)\)
\(\Leftrightarrow\left(2x+1\right)\left(x^2+x-2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-\frac{1}{2}\Rightarrow y=-\frac{1}{2}\\x=1\Rightarrow y=-2\\x=-2\Rightarrow y=1\end{matrix}\right.\)