1. 6.2 Sum and Difference Formulas
Objective
To develop and use formulas for the trigonometric functions of a sum or difference of two angle measures
Yep, they sure are!
Ain’t they a silly bunch?

2. What’s the point?

3. For example…
sin(15) = sin( )
cos(75) = cos( )
Now we just need to know what this means when we use the sum and difference formulas

4. Sum and Difference Formulas for Cosines
Sum and Difference Formulas for Sines

5. Sum and Difference Formulas for Sines
sin
cos
cos
sin
sin
cos
cos
sin

6. Sum and Difference Formulas for Cosines
cos
cos
sin
sin
cos
cos
sin
sin

7. Can you memorize these formulas
Can you memorize these formulas? You will have to if you take college trigonometry. Here is a love story to help introduce the trigonometry sum and difference formulas in an interesting way:

8. As we all know, some of the people to whom we are attracted are not attracted to us. And it is not unusual for a person who has shown interest in us to later lose interest in us. Maybe that is a good thing, because it forces us to date a lot of people and to become more experienced in maintaining relationships.

9. Anyway, this is the story of Sinbad and Cosette
Anyway, this is the story of Sinbad and Cosette. Sinbad loved Cosette, but Cosette did not feel the same way about Sinbad.

10. Naturally, when Sinbad was in charge of their double date, he put himself with Cosette, and he put her sister with his brother:
sin(A + B) = sin A cosB + cosA sinB.
sin(A - B) = sin A cosB - cosA sinB.
Sinbad loved to tell people that his and Cosette's signs were the same.

11. cos(A + B) = cosA cosB - sinA sinB.
However, when Cosette was in charge of the double date she placed herself with her sister and put Sinbad with his brother. She made sure everyone knew that their signs were NOT the same:
cos(A + B) = cosA cosB - sinA sinB.
cos(A - B) = cosA cosB + sinA sinB.
Also, notice that Cosette placed herself and her sister BEFORE Sinbad and his brother. This detail was important to Cosette. She was very snobby, you know.

12. Finding exact values of trig expressions
Split the given number into the sum/difference of unit circle values we know
Change the problem using the correct formula
Simplify by replacing in trig values

13. cos(A + B) = cosA cosB - sinA sinB. 1. Split 75o into 30o and 45o
1. Split 75o into 30o and 45o
2. Use the cosine formula
cos(A + B) = cosA cosB - sinA sinB.

14. You should memorize this
You should memorize this. This is a great reference because you can figure out the trig functions of all these angles quickly.

15. cos(A + B) = cosA cosB - sinA sinB. 3. Replace with Trig values cos
cos(A + B) = cosA cosB - sinA sinB.
3. Replace with
Trig values
cos
sin
cos
sin

16. cos(A + B) = cosA cosB - sinA sinB.
cos(A + B) = cosA cosB - sinA sinB.

18. Look at the formulas. 
Which one does it match?

19. Find the exact value of:

20. You will need to know these formulas so let's study them a minute to see the best way to memorize them.
opposite
cos has same trig functions in first term and in last term, but opposite signs between terms.
same
sin has opposite trig functions in each term but same signs between terms.

21. Verifying Identities
These three steps are key in verifying identities that require the sum and difference formulas: 1. Write in expanded form 2. Substitute known values 3. Simplify

22. Verifying Identities

23. We will work with the left side.

25. Sum and Difference Formulas for Tangent

26. Find tan 105° tan 105° = tan ( 60° + 45°) tan 60° + tan 45° =

27. Find tan 105°