1. 9.2 - 9.3 The Law of Sines and The Law of Cosines
MAT 204 F08
The Law of Sines and The Law of Cosines
In this chapter, we will work with oblique triangles  triangles that do NOT contain a right angle.
An oblique triangle has either:
three acute angles
two acute angles and one obtuse angle or

2. Every triangle has 3 sides and 3 angles.
MAT 204 F08
Every triangle has 3 sides and 3 angles.
To solve a triangle means to find the lengths of its sides and the measures of its angles.
To do this, we need to know at least three of these parts, and at least one of them must be a side.

3. Here are the four possible combinations of parts:
MAT 204 F08
Here are the four possible combinations of parts:
Two angles and one side (ASA or SAA)
Two sides and the angle opposite one of them (SSA)
Two sides and the included angle (SAS)
Three sides (SSS)

4. Two angles and one side (ASA or SAA)
MAT 204 F08
Case 1:
Two angles and one side (ASA or SAA)

5. Two sides and the angle opposite one of them (SSA)
MAT 204 F08
Case 2:
Two sides and the angle opposite one of them (SSA)

6. Two sides and the included angle (SAS)
MAT 204 F08
Case 3:
Two sides and the included angle (SAS)

7. MAT 204 F08
Case 4:
Three sides (SSS)

8. The Law of Sines Three equations for the price of one! MAT 204 F08 C b

9. Solving Case 1: ASA or SAA
Give lengths to two decimal places.

10. Solving Case 1: ASA or SAA
Give lengths to two decimal places.

11. MAT 204 F08
Solving Case 2: SSA
In this case, we are given two sides and an angle opposite.
This is called the AMBIGUOUS CASE. That is because it may yield no solution, one solution, or two solutions, depending on the given information.

12. SSA --- The Ambiguous Case

13. No Triangle If , then side is not sufficiently long enough to form a triangle.

14. One Right Triangle If , then side is just long enough to form a right triangle.

15. Two Triangles If and , two distinct triangles can be formed from the given information.

16. One Triangle If , only one triangle can be formed.

17. Give lengths to two decimal places and angles to nearest tenth of a degree.

18. Continued from above

19. Give lengths to two decimal places and angles to nearest tenth of a degree.

20. Continued from above

21. Give lengths to two decimal places and angles to nearest tenth of a degree.

22. Making fairly accurate sketches can help you to determine the number of solutions.

23. Example: Solve ABC where A = 27.6, a =112, and c = 165.
Give lengths to two decimal places and angles to nearest tenth of a degree.

24. Continued from above

25. To deal with Case 3 (SAS) and Case 4 (SSS), we do not have enough information to use the Law of Sines.
So, it is time to call in the Law of Cosines.

26. B A C c b a
The Law of Cosines

27. Using Law of cosines to Find the Measure of an Angle
*To find the angle using Law of Cosines, you will need to solve the Law of Cosines formula for CosA, CosB, or CosC.
For example, if you want to find the measure of angle C, you would solve the following equation for CosC:
To solve for angle C, you would take the cos-1 of both sides.

28. Guidelines for Solving Case 3: SAS
When given two sides and the included angle, follow these steps:
Use the Law of Cosines to find the third side.
Use the Law of Cosines to find one of the remaining angles. You could use the Law of Sines here, but you must be careful due to the ambiguous situation. To keep out of trouble, find the SMALLER of the two remaining angles (It is the one opposite the shorter side.)
Find the third angle by subtracting the two known angles from 180.

29. Solving Case 3: SAS
Example: Solve ABC where a = 184, b = 125, and C = 27.2.
Give length to one decimal place and angles to nearest tenth of a degree.

30. Continued from above

31. Solving Case 3: SAS
Example: Solve ABC where b = 16.4, c = 10.6, and A = 128.5.
Give length to one decimal place and angles to nearest tenth of a degree.

32. Continued from above

33. Guidelines for Solving Case 4: SSS
When given three sides, follow these steps:
Use the Law of Cosines to find the LARGEST ANGLE (opposite the largest side).
Use the Law of Sines to find either of the two remaining angles.
Find the third angle by subtracting the two known angles from 180.

35. Solving Case 4: SSS
Example: Solve ABC where a = 128, b = 146, and c = 222.
Give angles to nearest tenth of a degree.

36. Continued from above

37. (Let bold red represent the given info)
When to use what……
(Let bold red represent the given info)
SAS
AAS
ASA
Be careful!! May have 0, 1, or 2 solutions.
SSS
SSA
Use Law of Sines
Use Law of Cosines

39. Give lengths to two decimal places and angles to nearest tenth of a degree.

40. Continued from above

41. MAT 204 F08
End of Sections 9.2 – 9.3