The double angle formula is a very practical type of formula in trigonometric functions. It expresses the trigonometric functions of the double angle using the trigonometric functions of the original angle. It can be used to simplify calculation formulas and reduce the number of times trigonometric functions are calculated. It is also widely used in engineering.

sinαsinβ=-1/2[cos(α+β)-cos(α-β)]

cosαcosβ=1/2[cos(α+β)+cos(α-β)]

sinαcosβ=1/2[sin(α+β)+sin(α-β)]

cosαsinβ=1/2[sin(α+β)-sin(α-β)]