code x
sinax cosbx = 1/2 [sin(a-b)x  + sin(a+b)x]
cosax cosbx = 1/2 [cos(a-b)x + cos(a+b)x]
sinax sinbx =  1/2 [cos(a-b)x -  cos(a+b)x]
sin2x = 1/2 [1 – cos(2x)]
cos2x = 1/2 [1 + cos(2x)]