Answer
$$1$$
Work Step by Step
Apply the sum formula:
$\tan(A+B)=\dfrac{\tan A + \tan B}{1-\tan A \ \tan B }$
Therefore,
$$ \dfrac{\tan 20^\circ + \tan 25^\circ}{1-\tan 20^\circ \ \tan 20^\circ } =\tan (20^\circ +25^\circ) \\
=\tan \ (45^\circ) \\
=1$$
