If I times that fraction by

, I am actually just multiplying by one. But look at what happens:

So the rule is:

For any fraction multiply by to give

For the purpose of your question, this is how you rationalize the denominator., but for a harder example, you may read on.

However, it gets harder if you have something such as:

In this scenario, you have to use the 'difference of two squares' to rationalize, by making the denominator a difference of two squares, which is where . So apply that to the question, where and b=4:

Note the 'difference of two squares' in the denominator? Using the difference of two squares rule above, we can simplify this to: