1 - 2sin2(π4+θ) =
cos2θ
-cos2θ
sin2θ
-sin2θ
Have we see an expression similar to 1 - 2sin2(π4+θ) before?
Yes,
cos2x = 1 - 2sin2x
Here x = π4+θ
⇒ 1 - 2sin2(π4+θ) = cos2(π4+θ)
= cos(π4+2θ)
= −sin2θ