Math Trig Prove: TanB= |m1-m2/ 1+m1m2|

There are two non-perpendicular, non-vertical lines that form an angle, B, show that this is true:
TanB= |m1-m2/ 1+m1m2|

follows from tan [ Θ - Φ ] = { tan Θ - tan Φ } / [ 1 + tan Θ tan Φ ]

which is proven by sin [ Θ - Φ ] / cos [Θ - Φ ] , formulas which you should know