Thinking Mathematically (6th Edition)

Rationalizing a denominator means you rewrite a radical expression so that there is no radical in its denominator. This can be done by multiplying a radical expression to both the numerator and the denominator so that the radical in the denominator becomes a perfect square. To simplify $\dfrac{2}{\sqrt5}$, multiply $\sqrt5$ to both the numerator and the denominator as it will make the denominator become $\sqrt{25}$, which is equal to $5$. To illustrate: \begin{align*} \frac{2}{\sqrt5}&=\frac{2}{\sqrt5} \cdot \frac{\sqrt5}{\sqrt5}\\\\ &=\frac{2\sqrt5}{\sqrt{25}}\\\\ &=\frac{2\sqrt5}{5} \end{align*}