In this lesson, students prove that the length of the arc intercepted by a central angle is proportional to the radius of the circle. Then, they learn that the ratio of arc length to radius is called the radian measure of an angle. They use string to measure arc length in terms of a circle’s radius, and connect the results to the definition of radian angle measurement.
An understanding of radian measure will be necessary in future courses when students extend the domain of trigonometric functions using the unit circle.
Students have an opportunity to reason abstractly (MP2) as they consider the connection between radian measurements and arc lengths.
Lesson overview
 11.1 Warmup: A OneUnit Radius (5 minutes)
 11.2 Activity: A Constant Ratio (15 minutes)

11.3 Activity: Defining Radians (15 minutes)
 Includes "Are you Ready for More?" extension problem
 Lesson Synthesis
 11.4 Cooldown: Find a Radian Measure (5 minutes)
Learning goals:
 Comprehend (in spoken and written language) that the radian measure of an angle whose vertex is the center of a circle is the ratio of the length of the arc defined by the angle to the circle’s radius.
Learning goals (student facing):
 Let’s look at a new way to measure angles.
Learning targets (student facing):
 I know that the radian measure of an angle whose vertex is the center of a circle is the ratio of the length of the arc defined by the angle to the circle’s radius.
Required materials:
 Scissors
 String
Required preparation:

Be prepared to display an applet for all to see during the lesson synthesis.
Glossary:
 radian  The radian measure of an angle whose vertex is at the center of a circle is the ratio between the length of the arc defined by the angle and the radius of the circle.
 Access the complete Geometry Course Glossary.
Standards:
 This lesson builds on the standard: CCSS.HSGC.A.1MS.GC.1MO.G.C.A.1
 This lesson builds towards the standard: CCSS.HSGC.B.5MS.GC.5MO.G.C.B.4MO.G.C.B.5
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