Đáp án:
`thaipro`
`x \ne -9`
`x^2+(81x^2)/(x+9)^2=40`
`⇔x^2(x+9)^2+81x^2=40(x+9)^2`
`⇔x^2(x^2+18x+81)+81x^2=40(x^2+18x_81)`
`⇔x^4+18x^3+81x^2+81x^2=40x^2+720x+3240`
`⇔x^4+18x^3+122x^2-720x-3240=0`
`⇔(x^4-2x^3-18x^2)+(20x^3-40x^2-360x)+(180x^2-360x-3240)=0`
`⇔(x^2-2x-18)(x^2+20x+180)=0`
`⇔`\(\left[ \begin{array}{l}x^2-2x-18=0\\x^2+20x+180=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=1+\sqrt19\\x=1-\sqrt19\end{array} \right.\)
