Dear experts,

I am practicing the Trig identity, however met some issue in the changing $\displaystyle {\2tanB/(1+tanB)} TO {\1-3tan^2 B} $

Please kindly advise me:

identify: $\displaystyle \tan3B= \dfrac{\3tanB - tan^2 B }{\1-3tan^2 B}LHS := \dfrac{\ tanB + tan2 B }{\1-tanB.tan2B} = \dfrac{\3tanB - tan^2 B }{\2tanB/(1+tanB)}$

Thank you so much!