Proof of sum to product formula
From the product to sum formula:
2sin Asin B = cos (A + B) − cos(A − B)
Let A + B = C A − B = D Adding 2A = C + D A = Subtracting 2B = C − D B =
Let A + B = C A − B = D
Adding 2A = C + D
A =
Subtracting 2B = C − D
B =
Substituting into: 2sin Asin B = cos (A + B) − cos(A − B)
cos C − cos D = 2csin sin
Close Window