# Trig Sum and Difference Identities

Taught by YourMathGal
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Lesson Summary:

In this lesson, Julie Harland explains how to derive the formulas for the cosine of the difference of two angles, the sine of the sum of two angles, and the sine of the difference of two angles. She uses the formula for the cosine of the sum of two angles and a few other identities to derive these formulas. The lesson includes examples using the formulas derived.

Lesson Description:

Uses the formula for the cosine of the sum of two angles to derive the formulas for the cosine of the difference of two angles, the sine of the sum of two angles, and the sine of the difference of two angles. Examples using the formula are provided.

More free YouTube videos by Julie Harland are organized at http://yourmathgal.com

• What is the trig formula for cos(A - B)?
• What is the trig formula for cos(A + B)?
• What is the trig formula for sin(A - B)?
• What is the trig formula for sin(A + B)?
• What are the trig sum and difference identities?
• How can you derive the formulas for sin and cos of the sum and difference of angles?
• How can you use the formula for cos(A + B) to find the formula for cos(A - B)?
• Why does sin(A + B) = cos(90 - A - B)?
• How can you remember the four trig sum and difference of angles formulas?
• How can you find use the angle formulas in example right triangle problems?
• #### Staff Review

• Currently 4.0/5 Stars.
This lesson does a great job of not only showing what the formulas are for the trig identities of cos and sin of the sum and difference of two angles, but it also shows how to derive each of them if you know the formula for cos(A + B). Each derivation is shown and explained step by step. Also, these formulas are used in example right triangles.