First, students will learn what the law of cosines is. Next, students will see a visual proof of the law of cosines.

Law Of Cosines I (visual proof) by Micro Visual Proofs

After that, students will see the classic geometric proof of the law of cosines.

The Cosine Rule (1 of 3: Proof of the Formula) by Eddie Woo

Next, students will learn how to use the law of cosines to solve triangles. That is, students will learn how to determine all the unknown angles and side lengths of a triangle using the law of cosines, given SSS or SAS. They'll learn how to solve both word problems and mathematical ones. Here's a word problem involving a missing side, and here's another. Here's a word problem involving a missing angle. Conclude by giving your students this challenge.

Conclude by giving your students these challenges:

- Pegboard Quads by NRICH
- Triangle in a Triangle by NRICH
- Hypotenuse Lattice Points by NRICH
- Oh for the Mathematics of Yesteryear by NRICH
- 2019 AMC 8, Problem 17