Answer
$0$
Work Step by Step
RECALL:
$\tan \alpha =\dfrac {\sin \alpha }{\cos \alpha }$
Thus,
$\tan \alpha -\dfrac {\sin \alpha }{\cos \alpha }
\\=\dfrac {\sin \alpha }{\cos \alpha }-\dfrac {\sin \alpha }{\cos \alpha }
\\=0$
Therefore,
$\tan 40^o-\dfrac {\sin 40^o}{\cos 40^o}=0$
