tan (a-b) = tan a - tan b/1 + tan a × tan b

Identitas trigonometri.

[tex]\displaystyle \tan (a-b)=\frac{\sin (a-b)}{\cos (a-b)}\\ =\frac{\sin a \cos b-\cos a \sin b}{\cos a \cos b+\sin a \sin b}\\ =\frac{\frac{\sin a \cos b-\cos a \sin b}{\cos a \cos b}}{\frac{\cos a \cos b+\sin a \sin b}{\cos a \cos b}}\\ =\frac{\tan a-\tan b}{1+\tan a \tan b}[/tex]
Terbukti.