Precalculus: Law of Cosines

Objectives

• Students will be able to state the Law of Cosines.
• Students will be able to apply the Law of Cosines to find missing sides and angles in a triangle.
• Students will understand why the Law of Cosines is true.

Vocabulary

• cosine of an angle
• obtuse angle
• acute angle
• right angle

About the Lesson

This lesson involves visualizing and exploring the Law of Cosines.
As a result, students will:

• Manipulate a triangle to observe the equality of the Law of Cosines.
• Manipulate a triangle to discover why the Law of Cosines is true.
• Consider the Law of Cosines in connection with previous knowledge about right triangle trigonometry.
• Determine when the Law of Cosines can be used to solve for unknown side lengths and angle measures in a triangle, and apply it to solve when possible.