9-4

Law of Cosines

This week in advanced math we finish up chapter 9. We didn’t really learn anything new because exams are next week. We began to review chapters 7, 8, and 9 for our exam next Thursday. Chapter 9 was probably the easiest of the three. The entire chapter was about solving all kinds of different triangles. Law of Cosines was one of the easiest sections in chapter 9. You use law of cosines when you cannot use SOHCAHTOA or law of sines. All you have to do to solve the triangle is plug in the formula:

(opposite leg)2 = (adjacent leg)2 + (adjacent leg)2 – 2 (adjacent leg) (adjacent leg) cos(angle b/w)

Example 1:

Triangle ABC a = 2 b = 6 and angle C = 80°
1. Draw your triangle and label your sides.
2. Plug into your formula c2 = 22 + 62 – 2 (2)(6) cos 80°
3. Square root everything and plug it into your calculator
4. Your answer will be 5.99