The Sum and Difference Formulae
The Sum and Difference Formulae are also known as the Compound Angle Formulae. Now, without further adieu, Trig Frog presents, The Formulae:
sin(A + B) = sinA cosB + cosA sinB
sin(A - B) = sinA cosB - cosA sinB
cos(A + B) = cosA cosB - sinA sinB
cos(A - B) = cosA cosB + sinA sinB
tan(A + B) = (tanA + tanB)/(1 - tanA tanB)
tan(A - B) = (tan A - tanB)/(1 + tanA tanB)
The proofs for the Sum and Difference Formulae are some of the most complicated proofs that Trig Frog has ever seen. But, since thou wishes to know them, Trig Frog will provide you with a link to a page that has the proofs nicely explained and illustrated: Acts Networks
So, why shouldst thou take the trouble to learn the Sum and Difference Formulae? Trig Frog says: The Sum and Difference Formulae of trigonometry can be used to determine the exact values of trigonometric functions involving sums and differences of special angles, to solve trigonometric equations, or to verify trigonometric identities. They can even be used to prove some of the Multiple Angle Formulae! Now then, try thy hand at Trig Frog's sample problems.
