The law of cosines is a statement about a general triangle that relates the lengths of its sides to the cosine of one of its angles.

The law of cosine is used to solve triangles that are not right-angled. Such triangles are called oblique triangles.

Let and be the lengths of the legs of a triangle opposite angles and .

Then the law of cosines states:

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Let and be the lengths of the legs of a triangle opposite angles and .

Then the law of cosines states:

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