Watch this video to learn about the rules of surds. For example, \(\sqrt 4 = 2\) is not a surd. However \(\sqrt 5\) is a surd. If you use a calculator, you will see that \(\sqrt 5 = 2.236067977..

Rationalising an expression means getting rid of any surds from the bottom (denominator) of fractions. Usually when you are asked to simplify an expression it means you should also rationalise it.