\[x + 81x^{- 1} = 18\ \ \ \ | \cdot x\]
\[x^{2} + 80 = 18x\]
\[x^{2} - 18x + 80 = 0\]
\[D_{1} = 81 - 80 = 1\]
\[x_{1} = 9 + 1 = 10;\]
\[x_{2} = 9 - 1 = 8.\]
\[Ответ:x = 8;x = 10.\]