PT \(\Leftrightarrow12x^2+12x+1=0\)
\(\Leftrightarrow...\)
\(\left(x+2\right)^3-x^3+6x^2=7\)
\(\Leftrightarrow x^3+6x^2+12x+8-x^3+6x^2=7\)
\(\Leftrightarrow12x^2+12x+1=0\)
\(\Leftrightarrow12\left(x^2+2.\frac{1}{2}x+\frac{1}{4}-\frac{1}{4}+\frac{1}{12}\right)=0\)
\(\Leftrightarrow\left(x+\frac{1}{2}\right)^2-\frac{1}{6}=0\)
\(\Leftrightarrow\left(x+\frac{1}{2}\right)^2=\frac{1}{6}\)
⇔ \(x=\pm\sqrt{\frac{1}{6}}-\frac{1}{2}\)