I have three problems, and none of them seem to work. they give us a hint to use trig identities.

48) dy/dx = sec^4 (x)

50) dy/dx = 4 cos^2 (x)

52) dy/dx = (cos^4 (x) - sin^4 (x))

I have no idea how to do any of these. I spent the last 50 minutes trying