Question

(1+tan)(tan2x)=2tanx/1-tanx

Answer 1

is it : 1+tan(x)?

Answer 2

after simplifying you'll have 1=1...
what do you need to do...?

Answer 3

both sides have to be equal to each other

Answer 4

how did you simplify it to 1=1?

Answer 5

tan2x=2tanx/(1-tanx^2) - double-angle formula
so,
(1+tanx)*2*tanx/(1-tanx^2) = 2tanx/(1-tanx)
simplify - 2tans=canceled out on both sides
(1+tanx)/[(1-tanx)*(1+tanx)]= 1/(1-tanx)
everything canceling out....
so, in general left side of your original eq = right side
unless, I mis-read your original problem, it should be right...