Lesson 6

A Special Point

Lesson Narrative

In this lesson, students explore properties of angle bisectors. To build intuition, students first observe that pouring salt on a triangle forms ridges that meet at a peak, and the ridges appear to be angle bisectors. Students go on to prove that a point is on an angle bisector if and only if it is equidistant from the rays that form the angle. Then, they show that all 3 angle bisectors of a triangle meet at a single point, the incenter of the triangle. This will lead to constructing a triangle’s inscribed circle in a subsequent lesson.

Students create viable arguments (MP3) when they use what they know about triangle congruence to prove facts about angle bisectors.

Learning Goals

Teacher Facing

• Prove (using words and other representations) that the angle bisectors of a triangle are concurrent.

Student Facing

• Let’s see what we can learn about a triangle by watching how salt piles up on it.

Required Preparation

If desired, prepare a plate, bottle, container of salt, and a triangle made out of cardboard for the salt demonstration in the warm-up. Alternatively, prepare a method to show the embedded video for this activity.

The activity Point and Angle includes a digital and print version of the launch. For the digital version, be prepared to display an applet for all to see.

Student Facing

• I can explain why the angle bisectors of a triangle meet at a single point.
• I know any point on an angle bisector is equidistant from the rays that form the angle.

Building On

Building Towards

Glossary Entries

• incenter

The incenter of a triangle is the intersection of all three of the triangle’s angle bisectors. It is the center of the triangle’s inscribed circle.