Sum and Difference Formulas for Sine and Cosine
Have you ever found it unsettling that you can find the exact values of trigonometric functions for special angles but not, for example, for the angles of 15º or 75º? This is exactly what the identities that we are going to introduce do—they help to expand the pool of angles for which you can find the exact values of trigonometric functions.
These new identities are called the sum and difference formulas. These formulas allow you to break up the original angles into two special angles and find the function values of the original angle by using the function values of two special angles.
Let’s consider several examples of how you can break some of the angles into the sums or differences of two special ...