word still passed the sum of cosine alpha + cosine beta = 2 cos (alpha + beta) / 2, cos [(alpha beta) / 2]

"the sum of the mouth for sin sin alpha + sine beta = 2 (alpha + beta) / 2, cos [(alpha beta) / 2]

the difference between the word of negative game "cosine alpha cos beta 2 sin = - [(alpha + beta) / 2], sin [(alpha beta) / 2]

the mouth of the difference between the event ended the sine of alpha sine beta = 2 cos (alpha + beta) / 2, sin [(alpha beta) / 2]