## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$-\dfrac{\sqrt{3}}{3}$
The tangent is an odd function so $\tan{(- \theta)}= - \tan{\theta}$ Thus, $\tan{(-30 ^{\circ})} = -\tan{(30^{\circ})}= \boxed{-\dfrac{\sqrt{3}}{3}}$