1.
tan^2(A) (1- sin^2(A)) = tan^2(A)(cos^2(A)) = sin^2(A)
2. 4tan^2(A)(1-sin^2(A)) = 1
4sin^2(A) = 1
sin^2(A) = 1/4
sin (A) = +_1/2
A = pi/6, 5pi/6, 7pi/6 or 11pi/12 (which are 30 degrees, 150 degrees, 210 degrees and 330 degrees)
My question: find the derivative of sec^2(x)