\(9x^2+6x+1=81\)
\(\Rightarrow9x^2+3x+3x+1=81\)
\(\Rightarrow3x\left(3x+1\right)+\left(3x+1\right)=81\)
\(\Rightarrow\left(3x+1\right)^2=81\)
\(\Rightarrow\left[{}\begin{matrix}3x+1=9\\3x+1=-9\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=\dfrac{8}{3}\\x=-\dfrac{10}{3}\end{matrix}\right.\)




Hơi dài chút , giúp mình với nhé !

