41. 1)cos40cos20-sin40sin20 = cos(40°+20°) = cos60° = 1/2
2)cos70cos40+sin70sin40 = cos(70°-40°) = cos30° = √3/2
42. 1)cos5xcos2x+sin5xsin2x = cos(5x-2x) = cos3x
2)cos3xcosx-sin3xsinx = cos(3x+x) = cos4x
3)cosBsin5B-sinBcos5B = sin(5B-B) = sin4B
4)sin3acos2a+sin2acos3a = sin(3a+2a) = sin5a
43. 1) sin(x+y) - cosxsiny = sinxcosy+cosxsiny - cosxsiny = sinxcosy