解:$(1)\frac {\sqrt 7}{\sqrt 2}=\frac {\sqrt {14}}2$
$ (2)\frac 2{\sqrt {18}}=\frac 2{3\sqrt 2}=\frac {\sqrt 2}3$
$ (3)\frac 3{\sqrt {6x}}=\frac {3\sqrt {6x}}{6x}=\frac {\sqrt {6x}}{2x}$
$ (4)\frac 2{\sqrt {2a^2b}}=\frac {2×\sqrt {2a^2b}}{2a^2b}=\frac {\sqrt {2b}}{ab}$