Students will learn what the triangle angle bisector theorem is, why it's true, and how to use it. Before attempting this section, students must know the side splitter theorem, the converse of the isosceles triangle theorem, and how to solve proportions.

Here's the easiest way to prove the theorem:

Prove: Angle Bisector Theorem by LetsSolveMathProblems

And here's a slightly harder way:

Triangle angle bisector theorem proof by dangarbo10

Find \(x{:}\)

Conclude by giving your students these challenges:

- 2018 Math Kangaroo Levels 9-10 Problem #23 by STEM4all
- Gibraltar Geometry by NRICH
- Farey Sequences by NRICH
- The Number Jumbler by NRICH
- Birds & Bees by Pierce School: Problem / Solution