c.
\(x^3+8-\left(x+2\right)\left(x-4\right)=0\)
\(\Leftrightarrow\left(x+2\right)\left(x^2-2x+4\right)+\left(x+2\right)\left(x-4\right)=0\)
\(\Leftrightarrow\left(x+2\right)\left(x^2-x\right)=0\)
\(\Leftrightarrow\left(x+2\right)x\left(x-1\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=-2\\x=0\\x=1\end{matrix}\right.\)
d.
\(3x^2+8xy+5y^2=3x^2+3xy+5xy+5y^2\)
\(=3x\left(x+y\right)+5y\left(x+y\right)=\left(3x+5y\right)\left(x+y\right)\)
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