Proof: cos(a+b) = (cos a)(cos b)-(sin a)(sin b)

Lecture Description

Proof of the trig identity: cos(a+b) = (cos a)(cos b)-(sin a)(sin b).

Course Description

Videos on trigonometry. Watch the "Geometry" playlist first if you have trouble understanding the topics covered here.

Course Index

1. Basic Trigonometry Part 1
2. Basic Trigonometry Part 2
3. Radians and Degrees
4. Using Trig Functions Part 1
5. Using Trig Functions Part 2
6. The Unit Circle Definition of Trigonometric Functions Part 1
7. The Unit Circle Definition of Trigonometric Functions Part 2
8. Graph of the Sine Function
9. Graph of the Trig Function
10. Graphing Trig Functions
11. Trig Graphs
12. Determining the Equation of a Trigonometric Function
13. Trigonometric Identities Part 1
14. Proof: sin(a+b) = (cos a)(sin b) + (sin a)(cos b)
15. Proof: cos(a+b) = (cos a)(cos b)-(sin a)(sin b)
16. Trigonometric Identities Part 2
17. Trigonometric Identities Part 3
18. Trigonometry Word Problems Part 1
19. Trigonometry Word Problems Part 2
20. Law of Cosines
21. Navigation Word Problem
22. Proof: Law of Sines
23. Ferris Wheel Trig Problem Part 1
24. Ferris Wheel Trig Problem Part 2
25. Fun Trig Problem
26. Inverse Trig Functions: Arcsin
27. Inverse Trig Functions: Arctan
28. Inverse Trig Functions: Arccos
29. Trigonometry Identity Review/Fun