\ x= ^+_-\frac{1}{3}" alt="x^{-4}+81=18x^{-2}\\ \\ \frac{1}{x^4}+81=\frac{18}{x^2}\\ \\ \frac{1}{x^4}-\frac{18}{x^2}+81=0\\ \\ \frac{1-18x^2}{x^4}=-81\\ 1-18x^2=-81x^4\\ 81x^4-18x^2+1=0\\ \\ \\ \ x^2=t\\ \\ 81t^2-18t+1=0\\ (9t-1)^2=0\\ 9t-1=0\\ 9t=1\\ t=\frac{1}{9} \ => \ x= ^+_-\frac{1}{3}" align="absmiddle" class="latex-formula">