Derive the given equations by letting + = x and = y, which leads to = 1/2(x + y)

Derive the given equations by letting α + β = x and α − β = y, which leads to α = 1/2(x + y) and β = 1/2(x − y). The resulting equations are known as the factor formulas.

Use Eq. (20.16) and the substitutions above to derive the equation

Data from Eq. (20.16)

cos x cos y = -2 sin(x + y) sin(x - y) (20.20)