You're working on proving the identity 1−2sin2θcos3θ−sin3θ=cosθ+sinθ. Let's break down your steps to find the error.
Your initial steps are correct:
- You correctly expanded cos3θ and sin3θ using the triple angle formulas:
cos3θ=4cos3θ−3cosθ
sin3θ=3sinθ−4sin3θ
And the denominator using the double angle formula:
$1 - 2