What is the cosine rule?

What is it?

The cosine rule is a tool to find the size of an angle or the length of a side in a triangle.

When to use it?

- When we are given 2 sides and an angle

- When we are given 3 sides


For a triangle with sides a,b and c and angles A, B and C the cosine rule can be written as:

a2 = b2 + c2 - 2bc cos A


b2 = a2 + c2 - 2ac cos B


c2 = a2 + b2 - 2ab cos C

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