Cosine Rule

The Cosine rule, similar to the sine rule, is true for all triangles.

The Cosine rule states that, in any triangle abc,

This is rule is useful when:

You know all 3 sides. You know 2 sides and the angle at which they meet.

Example:

We can find A using the cosine rule:

cosA = 10^2 + 6^2 - 7^2                 2(10)(6)

cosA =  87             120

A = 43.53*

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