prove the trig identity
cos(4x) = 2-sec^2(2x)/sec^2(2x)
Recognize that
[2-sec^2(2x)]/sec^2(2x)
= 2 cos^2(2x) - 1
= cos^2(2x) - sin^2(2x)
and use the double angle identity.
cos(4x) = 2-sec^2(2x)/sec^2(2x)
[2-sec^2(2x)]/sec^2(2x)
= 2 cos^2(2x) - 1
= cos^2(2x) - sin^2(2x)
and use the double angle identity.