\$x^2+\frac{81x^2}{(x+9)^2}=40\$

crazyshou

\(\$x^2+\frac{81x^2}{(x+9)^2}=40\$利用配方法：\$(x-\frac{9x}{x+9})^2+2x\cdot\frac{9x}{x+9}=40\$，化为\$(\frac{x^2}{x+9})^2+18(\frac{x^2}{x+9})-40=0\$。\)