These 2 rules are used with non-right angle triangles. Which one to use will depend on the data you are given.

The Convention

Note: The convention on labelling the triangle is:
- Angles are in CAPITALS
- Sides are in lower case
In addition, each angle/side pair is such that 
- Angle A MUST BE opposite side a
- Angle B MUST BE opposite side b 
- Angle C MUST BE opposite side c 

The Sine Rule

Use this rule when you are given AN ANGLE AND ITS OPPOSITE SIDE.

Below are 2 examples using both versions of the sine rule.

The Cosine Rule

The rule can be written in the following two versions

2 scenarios when the Cosine rule is used

  1. When you have 2 sides and the angle between the 2 sides
  2. When you have all 3 sides of the triangle.

Watch this short video for examples of using the Cosine rule.

The full video lesson on the Sine & Cosine rules can be found here.
Triangles / Pythagoras
(R&F) Geometry - Parallel Lines (z-angles/f-angles/c-angles)