[tex]Use\\\\\sqrt{ab}=\sqrt{a}\cdot\sqrt{b}\\\\(a^n)^m=a^{nm}\\\\\sqrt{a^2}=|a|\\--------------------\\\\\sqrt{x^8y^{18}}=\sqrt{x^8}\cdot\sqrt{y^{18}}=\sqrt{x^{4\cdot2}}\cdot\sqrt{y^{9\cdot2}}=\sqrt{(x^4)^2}\cdot\sqrt{(y^9)^2}\\\\=|x^4||y^9|=\boxed{x^4|y|^9}[/tex]
