Question

cos c + cos d ≠ equal Cos (c+d)​

Answer 1

yes cos c+cos d≠cos(c+d)

Answer 2

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