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 * **How do we find the cosine of the difference of two angles and the cosine of the sum of two angles?** //(These formulas will be on the Regents Reference sheet)//

By the end of this lesson, you will be able to...
 * apply the formulas for cos(A - B) and cos(A + B) to find the exact value involving angles measured in radians and degrees
 * state the sum and difference formulas in words

Answer these two questions in your notebooks:


 * 1) What is the Cosine sum formula?
 * 2) What is the cosine difference formula?

[|Video 1]

[|Video 2]


 * **Classwork:** Now try these problems. Remember to use the exact measurement. Check your answers on your calculator.


 * 1) Find the exact value of cos 105° (Ans: (sqrt 2 - sqrt 6)/4)
 * 2) Find the exact value of cos 15° (Ans: (sqrt 6 + sqrt 2)/4)


 * **Homework:** Complete those that are multiples of three

[|Cosine_Sum.JPG]

[|Cosine_Difference.JPG]


 * **Answers:**

[|A-Cosine_Sum.JPG]

[|A-Cosine_Difference.JPG]


 * **Journal entry:** Explain why we cannot use the Distributive law to evaluate cos(A + B) as cosA + cosB.