In this lesson, students investigate circle sectors. A sector is a region of a circle enclosed between 2 radii. Students look for and express regularity in repeated reasoning (MP8) as they make sector area and arc length calculations and use their findings to create a general method for finding sector areas and arc lengths.
Building intuition about arc length and fractions of circles will be helpful as students develop a definition of radian measure in an upcoming lesson.
Lesson overview
 8.1 Warmup: Math Talk: Fractions of a Circle (5 minutes)

8.2 Activity: Sector Areas and Arc Lengths (15 minutes)
 Includes "Are you Ready for More?"extension problem
 8.3 Activity: Build a Method (15 minutes)
 Lesson Synthesis
 8.4 Cooldown: Use Your Method (5 minutes)
Learning goals:
 Generalize (orally and in writing) methods for calculating lengths of arcs and areas of sectors in circles.
Learning goals (student facing):
 Let’s analyze portions of circles.
Learning targets (student facing):
 I can calculate lengths of arcs and areas of sectors in circles.
Glossary:
 Sector  The region inside a circle lying between two radii of the circle.
 Access the complete Geometry Course Glossary.
Standards:
 This lesson builds on the standard: CCSS.7.G.B.4MS.7.G.4MO.7.GM.A.4b
 This lesson builds towards the standard: CCSS.HSGC.B.5MS.GC.5MO.G.C.B.4MO.G.C.B.5
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