\(x+y=-1\Rightarrow y=-1-x\)
Thế vào pt đầu:
\(x^3-\left(-1-x\right)^3=3\left(x+1+x\right)\)
\(\Leftrightarrow2x^3+3x^2-3x-2=0\)
\(\Leftrightarrow2\left(x^3-1\right)+3x\left(x-1\right)=0\)
\(\Leftrightarrow\left(x-1\right)\left(2x^2+5x+2\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=1\Rightarrow y=-2\\x=-2\Rightarrow y=1\\x=-\frac{1}{2}\Rightarrow y=-\frac{1}{2}\end{matrix}\right.\)