There is a hexagonal formula to recite, and then there is rote memorization. This formula is too simple.

cos(a+b)/2 = cos(π-c)/2 = cos(π/2-c/2)= sin(c/2)。 Can you understand this inference?

Simply put, SIN 30 = SIN (90-60) = COS60. See, this is the reason for the above formula. ...