Cosine multiple angle formula:1.cos2α = 2cos2α-1.2.cos2α =1? 2sin^2 α 3.cos2α = cos^2 α？ sin^2 α

Tangent dihedral formula: tan 2 α = 2 tan α/[1-(tan 2 α)] tan (1/2 * α) = (sin α)/(1+cos α) = (1-cos α)/sin α.

Power drop formula (half-angle formula): cos2 (a) = [1+cos2a]/2sin2 (a) = [1-cos2a]/2tan2 (a) = [1-cos2a]/[6544].