Question 681990
Prove that (1-tan^2 x) / (1+tan^2 x) = cos 2 x 
-----
Note: Since sin^2 + cos^2 = 1
Dividing by cos^2 you get: tan^2 + 1 = sec^2
-----
Using that Pythagorean relation you get:
(1-tan^2)/(sec^2) = cos(2x)
------
(1 - (sin^2/cos^2)/sec^2 = cos(2x)
(cos^2-sin^2) = cos(2x)
cos(2x) = cos(2x)