One thing I learned this week in Pre Calc 11 is how to find the side of a triangle using Cosine Law. The Cosine Law is used when you need to find a third side of a triangle. Tip: When trying to find a third angle of a triangle use the Alternate Cosine Law.

A trick to knowing when you are going to be using the Cosine Law is when you see a question asking to find the measurement of the third side of the triangle, and they give you the other two sides of the triangle and the angle between them.

Below is an example of a triangle where you need to find the third side. I will show how I determine what formula to use and how to solve…