Question

Double Angle Formula.
If the double angle formula for cos2(u) = 2cos^2(u)-1, then write the double angle formula for cos2(u) in terms of sine.

Answer 1

you should write it this way:
cos (2u) = 2cos^2 (u) - 1
but we know cos^2 u = 1 - sin^2 u

so cos (2u) = 2(1 - sin^2 u) - 1
= 2 - 2sin^2 u - 1
= 1 - 2sin^2 u

Answer 2

ohhhhh, i see!
thank you for making it simple to understand