tan70=tan20+2tan50

Prove it.

$\tan \left(70^{\circ}-20^{\circ}\right)=\frac{\tan 70^{\circ}-\tan 20^{\circ}}{1+\tan 70^{\circ} \tan 20^{\circ}}=\frac{\tan 70^{\circ}-\tan 20^{\circ}}{1+\operatorname{tab} 70^{\circ} \cdot \cot 70^{\circ}}=\frac{\tan 70^{\circ}-\tan 20^{\circ}}{1+1}$ $\therefore 2 \tan 50^{\circ}=\tan 70^{\circ}-\tan 20^{\circ} \therefore \tan 70^{\circ}=\tan 20^{\circ}+2 \tan 50^{\circ}($ Showed $)$