^2-(sinA)^2=2(cosA)^2-1=1-2(sinA)^2 (cosA)^2=(1+cos2A)/2 (sinA)^2=(1-cos2A)/2 sinαcosβ=[sin(α+β)+sin(α-β)]/2 cosαsinβ=[sin(α+β)-sin(α-β)]/2 cosαcosβ=[cos(α+β)+cos(α-β)]/2 sinαsinβ=-[cos(α+β)-cos(α-β)]/2