\(y^2\left(y-1\right)\left(y+1\right)=72\)
\(\Leftrightarrow y^2\left(y^2-1\right)=72\)
Đặt \(a=y^2\) ta được:
\(a\left(a-1\right)=72\)
\(\Leftrightarrow a^2-a=72\)
\(\Leftrightarrow a^2-a-72=0\)
\(\Leftrightarrow a^2-9a+8a-72=0\)
\(\Leftrightarrow a\left(a-9\right)+8\left(a-9\right)=0\)
\(\Leftrightarrow\left(a+8\right)\left(a-9\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}a=9\left(n\right)\\a=-8\left(l\right)\end{matrix}\right.\)
\(\Rightarrow y^2=9\)
\(\Leftrightarrow\left[{}\begin{matrix}y=3\\y=-3\end{matrix}\right.\)
