\(\left(3y+2\right)\left(3y-5\right)=8\) \(\Leftrightarrow9y^2+6y-15y-10=8\)
\(\Leftrightarrow9y^2-9y-18=0\) \(\Leftrightarrow y^2-y-2=0\)
\(\Leftrightarrow y\left(y-2\right)+y-2=0\) \(\Leftrightarrow\left(y+1\right)\left(y-2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}y+1=0\\y-2=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}y=-1\\y=2\end{matrix}\right.\)
Vậy ...
\(\left(3y+2\right)\cdot\left(3y-5\right)=8\)
\(\Leftrightarrow9y^2-9y-10=8\)
\(\Leftrightarrow9y^2-9y-\left(10+8\right)=0\)
\(\Leftrightarrow9y^2-9y-18=0\)
\(\Leftrightarrow9\cdot\left(y+1\right)\cdot\left(y-2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}y+1=0\\y-2=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}y=-1\\y=2\end{matrix}\right.\)
\(9y^2 -15y+6y-10=8\)
\(9y^2-9y=10+8\)
\(9y^2 -9y= 18\)
\(9y(y-1)=18\)
\(y(y-1)=2\)
\(y^2-y-2=0\)
\(y=2 \)
