tan^2 x= sin^2 x/cos^2x and cos^2 = 1-sin^2 x and Cos^2 x + sin^2 x = 1
therefore
1-(sin^2x/cos^2x) / 1+(sin^2x/cos^2x) then make the fractions of the top and bottom common by timeing by cosx therefore
(cos^2x--sin^2x) / ( Cos^2 x + sin^2 x)
therefore...
cos^2 x - sin^2 x /1 = cos^2 x - sin^2 x
cos^2 = 1-sin^2 therefore....
(1-sin^2 x) - sin^2 x = 1-2sin^2 x
tada~^^ lol