Download The Cosine Rule

The Cosine Rule
Non Right-Angled Triangle Trigonometry
By the end of this lesson you will be able to explain/calculate the
following:
1. An unknown length using the Cosine Rule
2. An unknown angle using the Cosine Rule
• The cosine rule is derived from a non-rightangled triangle divided into two right-angled
triangles
• In this case, Pythagoras’ theorem & the cosine
ratio are used to develop it.
• The triangle ABC in the figure below has been
divided into two right-angled triangles with base
sides equal to x and (c − x).