Chapter 5 - Number Theory and the Real Number System - 5.4 The Irrational Numbers - Concept and Vocabulary Check - Page 296: 10

$\sqrt3$

The rewrite the expression $\dfrac{5}{\sqrt{12}}$ so that there is no more radical in its denominator, $\sqrt3$ must be multiplied to both the numerator and the denominator so that when simplified, the denominator will end up having no radical anymore. This is because $\sqrt{36}=6$. Thus, the answer is $\sqrt3$.