# Math Help - prooof!!

1. ## prooof!!

can someone set me straight on this proof?

tan3A=(3tanA-tan^3)/(1-3tan^2A)

thanks a bunch

2. Originally Posted by ringer123
can someone set me straight on this proof?

tan3A=(3tanA-tan^3)/(1-3tan^2A)

thanks a bunch
Start from tan (A + 2A) and use the compound angle formula.

Replace tan(2A) using the double angle formula.

Simplify.