The half-angle identities are special cases of angle difference identities. To evaluate trigonometric functions of half an angle, the following identities can be applied. The sign of each formula is determined by the quadrant in which 2θ lies.
sin2θcos2θtan2θ=±21−cosθ=±21+cosθ=±1+cosθ1−cosθ
cos2θ=±21+cos(2⋅2θ)⇓cos2θ=±21+cosθ