Education Technology

Precalculus: Law of Cosines

by Texas Instruments


  • 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.



  • 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.