Generally, you don't want to have radical at the denominators. So, let's say that we want to simplify the expression #\frac{\sqrt{a}}{\sqrt{b}}#, where #a# and #b# can be any expression you want. Since, of course, #\frac{\sqrt{b}}{\sqrt{b}}=1#, we can multiply it without changing the value of our expression, so we have #\frac{\sqrt{a}}{\sqrt{b}}=\frac{\sqrt{a}}{\sqrt{b}} \cdot \frac{\sqrt{b}}{\sqrt{b}}#. The advantage is that now we observe that #\sqrt{b} \cdot \sqrt{b}=b#, and so our expression becomes #\frac\{\sqrt{ab}}{b}#, and we got rid of the radical at the denominator.