Câu 4 :
\(x+y+2=0\Rightarrow x+y=-2\)
\(D=x^2\left(x+y\right)-y^2\left(x+y\right)+2\left(x^2-y^2\right)+2\left(x+y\right)+3\)
\(D=\left(x+y\right)\left(x^2-y^2\right)+2\left(x^2-y^2\right)+2\left(x+y\right)+3\)
\(D=\left(x^2-y^2\right)\left(x+y+2\right)+2\left(x+y\right)+3\)
\(D=\left(x^2-y^2\right).0+2.\left(-2\right)+3=-1\)
\(\Rightarrow D.D=-1\)
Câu 5:
\(x+y+z=0\Rightarrow\left\{{}\begin{matrix}x+y=-z\\y+z=-x\\x+z=-y\end{matrix}\right.\)
\(M=2\left(x+y\right)\left(y+z\right)\left(x+z\right)=2.\left(-z\right).\left(-x\right).\left(-y\right)=-2xyz=-2.8=-16\)
\(\Rightarrow C.M=-16\)
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