sin3A = 3sinA-4(sinA)^3
(sinA-(3sinA-4(sinA)^3))/2sin2Acos2A=(4(sinA)^3-2sinA)/4sinAcosAcos2A=-2sinA(1-2(sinA)^2)/4sinAcosAcos2A=-cos2A/2cosAcos2A=-1/2cosA.
sin3A = 3sinA-4(sinA)^3
(sinA-(3sinA-4(sinA)^3))/2sin2Acos2A=(4(sinA)^3-2sinA)/4sinAcosAcos2A=-2sinA(1-2(sinA)^2)/4sinAcosAcos2A=-cos2A/2cosAcos2A=-1/2cosA.