Law of Cosines - Formula, Examples & Proof - [2-20-4]

View more at http://www.MathAndScience.com. In this lesson, you will learn about the law of cosines in trigonometry. We will focus on presenting the formula for the law of cosines, then we will work several examples to get practice, and we finish the lesson by proving the law of cosines. We can use the law of cosines for any triangle, including triangles that are arbitrary in shape and not right triangles. We can think of the law of cosines as being a modified Pythagorean theorem c^2 = a^2 + b^2 where we add an extra term to correct for it being a non-right triangle.