Circles

Lesson 12

Math

Unit 7

10th Grade

Lesson 12 of 14

Objective


Describe the proportional relationship between arc length and the radius of a circle. Convert between degrees and radians to write the arc measure in radians.

Common Core Standards


Core Standards

  • G.C.B.5 — Derive using similarity the fact that the length of the arc intercepted by an angle is proportional to the radius, and define the radian measure of the angle as the constant of proportionality; derive the formula for the area of a sector.

Foundational Standards

  • 7.RP.A.2

Criteria for Success


  1. Define radians as the number of radii needed to form the circumference. Because the measure of “radians” is not specific to the radius of the circle, the number of radians in a circle is a constant of $$2\pi$$.
  2. Define radians as the angle of turn of the arc so that you can describe the arc measure and central angle in both radians and degrees.
  3. Describe that the ratio of the circumference to the arc length is the same as the ratio of the central angle to the entire circle.
  4. Describe that as the radius increases, the arc length also increases proportionally but the arc measure stays constant. 
  5. Convert between radians and degrees. 

Tips for Teachers


A common misconception students may have about this objective may involve the ratio of the arc length to the entire circumference. 

Fishtank Plus

Unlock features to optimize your prep time, plan engaging lessons, and monitor student progress.

Anchor Problems

25-30 minutes


Problem 1

How many radii do you need to go around the complete circle?

Guiding Questions

Create a free account or sign in to access the Guiding Questions for this Anchor Problem.

References

GeoGebra CSS High School: Geometry (Circles)HSG.C.B.5, Movie: Radians to Revs

CSS High School: Geometry (Circles) by Tim Brzezinski is made available by GeoGebra under the CC BY-NC-SA 3.0 license. Copyright © International GeoGebra Institute, 2013. Accessed Sept. 19, 2018, 2:30 p.m..

Problem 2

Below are three circles with center $$A$$.

  • The smallest circle has a radius of $$\overline{AC}$$ with a length of 1 unit.
  • The next circle has a radius of $$\overline{AD}$$ with a length of 3 units.
  • The largest circle has a radius of $$\overline{AB}$$ with a length of 5 units. 

Are the radius lengths proportional to the arc lengths? Explain your reasoning. 

Guiding Questions

Create a free account or sign in to access the Guiding Questions for this Anchor Problem.

References

GeoGebra Geometry - 8.12 AP2

Geometry - 8.12 AP2 by Match Fishtank is made available by GeoGebra under the CC BY-NC-SA 3.0 license. Copyright © International GeoGebra Institute, 2013. Accessed June 13, 2017, 12:09 p.m..

Target Task

5-10 minutes


Below is circle $$A$$:

  1. What is the $$m\angle BAC$$ in degrees to the nearest hundredth?
  2. What is the $$m\widehat{BC}$$ in radians to the nearest hundredth? 

References

EngageNY Mathematics Geometry > Module 5 > Topic B > Lesson 10Exit Ticket, Question #1

Geometry > Module 5 > Topic B > Lesson 10 of the New York State Common Core Mathematics Curriculum from EngageNY and Great Minds. © 2015 Great Minds. Licensed by EngageNY of the New York State Education Department under the CC BY-NC-SA 3.0 US license. Accessed Dec. 2, 2016, 5:15 p.m..

Modified by Fishtank Learning, Inc.

Additional Practice


The following resources include problems and activities aligned to the objective of the lesson that can be used for additional practice or to create your own problem set.

  • Include problems where students are given the central angle and radius and asked to find the arc length.
  • Include problems where students are given the arc length and radius and asked to find the central angle.
  • Include problems such as “If the central angle of a circle with radius of $$2$$ inches has a measure of $$\frac{\pi}{6}$$ radians, what is the arc length? How many arcs of this radian measure are there in one circle? Does this change depending on the length of the radius?”
  • Include problems such as “Given a circle with a radius length and central angle measure of an arc, if you double the radius and halve the central angle, how will this affect the sector length?” 

Next

Calculate the sector area of a circle. Identify relationships between sector area, arc angle, and radius.

Lesson 13
icon/arrow/right/large

Lesson Map

A7CB09C2-D12F-4F55-80DB-37298FF0A765

Topic A: Equations of Circles

Topic B: Angle and Segment Relationships in Inscribed and Circumscribed Figures

Topic C: Arc Length, Radians, and Sector Area

Request a Demo

See all of the features of Fishtank in action and begin the conversation about adoption.

Learn more about Fishtank Learning School Adoption.

Contact Information

School Information

What courses are you interested in?

ELA

Math

Are you interested in onboarding professional learning for your teachers and instructional leaders?

Yes

No

Any other information you would like to provide about your school?

We Handle Materials So You Can Focus on Students

We Handle Materials So You Can Focus on Students

We've got you covered with rigorous, relevant, and adaptable math lesson plans for free