Students begin this lesson by discussing what they notice and wonder about several images of circles that contain different kinds of line segments. Then they are introduced to the vocabulary terms chord (a segment with both endpoints on a circle), central angle (an angle formed by two radii of a circle that each have an endpoint at the center of the circle), and arc (the portion of a circle between two endpoints). Students write definitions of these terms after analyzing examples and non-examples. Then they prove a property of congruent chords.

The definitions developed in this lesson are foundational for the rest of the unit. For example, students will use their knowledge of chords when analyzing inscribed angles.

Students attend to precision (MP6) as they write careful definitions of vocabulary terms.