Law of Cosines

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SWBAT use the Law of Cosines to solve oblique triangles.

Big Idea

A direct instruction lesson where we model for students the use of the Law of Cosines.


10 minutes

The warm-up question for today is located on Slide 1 of the PowerPoint Law of Cosines.

I want my students to continue to practice the ambiguous case of the Law of Sines by determining how many triangles would be created given each situation. Once we review the Law of Sines, I will connect this to the Law of Cosines. It is essential that students see that the Law of Sines cannot be used to solve all oblique triangles as we always need an angle and side pair. I want students to understand why these two Laws are necessary.

Explanation: Law of Cosines

35 minutes

Closure: 3-2-1 assessment

5 minutes

At the conclusion of this lesson, students have been exposed to all the major learning in this unit. So it is a great time for them to complete a 3-2-1 Assessment based on this Unit. Their responses can help us judge what to focus on during the review day this unit.


Tonight my students will begin to work on Homework 7 - Fundamentals of Trigonometry. The assignment provides the students with a lot of practice with the Laws of Sine and Cosine.