Answer
$0$
Work Step by Step
Recall the co-function identity:
$ \cos (\theta)=\sin(90^{\circ}-\theta)$ and $\tan \theta=\dfrac{\sin \theta}{\cos \theta}$
Therefore, $$\tan \ 20^{\circ} -\dfrac{\cos 70^{\circ}}{\cos 20^{\circ}}=\tan \ 20^{\circ} -\dfrac{\sin 20^{\circ}}{\cos 20^{\circ}}\\=\tan \ 20^{\circ} -\tan 20^{\circ}\\=0$$
