Added by John Pruess (TQ Team 20991) on March 03, 1999 at 02:16:21:
sin u + sin v = 2(sin((u + v)/2))(cos((u - v)/2))
sin u - sin v = 2(cos((u + v)/2))(sin((u - v)/2))
cos u + cos v = 2(cos((u + v)/2))(cos((u - v)/2))
cos u - cos v = -2(cos((u + v)/2))(sin((u - v)/2))