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It is a mathematical equation that allows you to "solve" a triangle (find all length and angle values), if you know 2 sides and an included angle, or all three sides. It doesn't have to be a right triangle. You can find the cosine on a calculator easily.

c2 = a2 + b2- 2ab cos C

C = included angle

c = side opposite angle C (c)

a = side a

b = side b
The cosine law relates the length of the sides of a triangle to one of the angles in the triangle. If the triangle is labelled with vertices A, B, C with usual notation for edges (ie a is the side opposite the vertex A, so not touching A) and if x is the angle at vertex C then the cosine law says (c^2)=(a^2)+(b^2)-2abcos(x)

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Q: What is the law of cosines?
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When you have a right angle what does the law of cosines reduce to?

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