\(\Leftrightarrow a^3+6a^2b+12ab^2+8b^3=6a^2b+12ab^2-6ab+1\)
\(\Leftrightarrow\left(a+2b\right)^3=6ab\left(a+2b-1\right)+1\)
\(\Leftrightarrow\left(a+2b\right)^3-1-6ab\left(a+2b-1\right)=0\)
\(\Leftrightarrow\left(a+2b-1\right)\left(\left(a+2b\right)^2+a+2b+1\right)-6ab\left(a+2b-1\right)=0\)
\(\Leftrightarrow\left(a+2b-1\right)\left(a^2+4ab+4b^2+a+2b+1-6ab\right)=0\)
\(\Leftrightarrow\left(a+2b-1\right)\left(a^2-2ab+4b^2+a+2b+1\right)=0\)
TH1: Nếu \(a+2b-1=0\)
\(\Leftrightarrow a+2b-1=0\)
\(\Rightarrow a+2b=1\)
TH2: \(a^2-2ab+4b^2+a+2b+1=0\)
\(\Leftrightarrow a^2-2ab+4b^2+a+2b+1=0\)
\(\Leftrightarrow\left(a-b+\frac{1}{2}\right)^2+3\left(b+\frac{1}{2}\right)^2=0\)
\(\Rightarrow\left\{{}\begin{matrix}a-b+\frac{1}{2}=0\\b+\frac{1}{2}=0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=-1\\b=-\frac{1}{2}\end{matrix}\right.\) \(\Rightarrow a+2b=-2\)
