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The Law of Cosines


In any given triangle a, b, and c  are the sides of the triangle and also opposite angles are A, B, and C.

The Law of Cosines

Then the Cosine Rule states

c2 = a2 + b2 − 2ab cos(C)

It is also called The Law of Cosines

Formulas (Versions for a, b and c)

According to the cosines law formula, to check the length of sides of triangle say △ABC, we can see below  as;

a2 = b2 + c2 − 2bc cos(A)

b2 = a2 + c2 − 2ac cos(B)

c2 = a2 + b2 − 2ab cos(C)

Find the angles of triangle

And if we want to calculate the angles of triangle, then the cosine rule is followed as;

angles of triangle