The Law of Cosines can be used to solve a triangle when either
(1)two sides and the included angle are known, or (2)three sides are
known. A statement of the Law of Cosines is given below:
For any triangle with sides a, b, and c and opposite angles
Letís look at an example that shows how to use the Law of Cosines.
Given two sides of a triangle
and the angle find the remaining parts of the triangle.
can use the Law of Cosines to find the missing side c. See figure
can use the Law of Cosines again to find the missing angles:
that , which is what we would expect (except for rounding
error) since the sum of the measures of the angles in a triangle should
always be 180į.