Depending on which information we are given about a triangle, we may also need to use the cosine rule. This formula should also be memorised:

Example

1. Find side length B

2. Find angle C

The teacher will show you how to derive this formula. You don’t need to be able to do this yourself, but it is good for you to see that the formula is logically arrived at and not just conjured out of thin air! Below, for your reference, is a summary of how to derive the formula:

Exercise

Let’s complete the following exercise:

The answers are below:

Applied Problems

Let’s use the sine rule and cosine rule in applied problems below.

Example

Exercise

Let’s complete exercise 11 from pages 230 and 231 of the extended textbook:

The answers are below: