Circles

Lesson 9

Math

Unit 7

10th Grade

Lesson 9 of 14

Objective


Construct tangent lines to a circle to define and describe the circumscribed angle.

Common Core Standards


Core Standards

  • G.C.A.2 — Identify and describe relationships among inscribed angles, radii, and chords. Include the relationship between central, inscribed, and circumscribed angles; inscribed angles on a diameter are right angles; the radius of a circle is perpendicular to the tangent where the radius intersects the circle.
  • G.C.A.4 — Construct a tangent line from a point outside a given circle to the circle.

Foundational Standards

  • G.CO.D.13

Criteria for Success


  1. Using the property that a radius is perpendicular to the tangent line at the point of tangency, identify the constructions necessary to construct a tangent line to a point on the circle.
  2. Define a circumscribed angle as an angle formed by two tangent lines that intersect a point outside the circle.
  3. Describe the relationship between a circumscribed angle and the central angle that meet at the two points of tangency as supplementary. See Unit 7 Glossary for a visual.
  4. Use properties of tangent lines to solve problems.

Tips for Teachers


  • This lesson addressed standard G-C.4 (+), which is represented in its fullest in the problem set guidance. Anchor Problem #1 asks students to construct a tangent line given a point on the circle, whereas G-C.4(+) states, “Construct a tangent line from a point outside a given circle to the circle.”
  • The following tools will be needed for this lesson: compass and straightedge. 
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Anchor Problems

25-30 minutes


Problem 1

Construct a tangent line to circle $$A$$ at point $$B$$.

Guiding Questions

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References

Math Open Reference Tangent to a Circle at a Point

Tangent to a Circle at a Point by John D. Page is made available on Math Open Reference. © 2011 Copyright Math Open Reference. All rights reserved. Accessed Sept. 19, 2018, 2:04 p.m..

Problem 2

Below is circle $$A$$ with tangent lines $$\overleftrightarrow{BD}$$ and $$\overleftrightarrow{CD}$$.

What is the relationship between the central angle and the circumscribed angle?

Guiding Questions

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References

GeoGebra Geometry - 8.9 AP2

Geometry - 8.9 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, 11:49 a.m..

Target Task

5-10 minutes


  1. Draw a circle tangent to both rays of this angle.

  1. Let $$B$$ and $$C$$ be the points of tangency of your circle. Find the measures of $$\angle ABC$$ and $$\angle ACB$$. Explain how you determined your answer.
  2. Let $$P$$ be the center of your circle. Find the measures of the angles in $$\triangle APB$$.

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.

Next

Use angle and side length relationships with chords, tangents, inscribed angles, and circumscribed angles to solve problems.

Lesson 10
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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

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