Question

Sin c - sin d = 2 cos (c+d/2).sin (c-d/2)

Answer 1

Trigonometric formulae:

sin C + sin D = 2 Sin (C + D)/2 cos (C - D)/2

sin C - sin D = 2 cos (C + D)/2 sin (C - D)/2

cos C + cos D = 2 cos (C + D)/2 cos (C - D)/2

cos D + cos C = 2 sin (C + D)/2 sin (C - D)/2