Circles

Lesson 8

Math

Unit 7

10th Grade

Lesson 8 of 14

Objective


Define and determine properties of tangents and secants of circles to solve problems with inscribed and circumscribed triangles.

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.3 — Construct the inscribed and circumscribed circles of a triangle, and prove properties of angles for a quadrilateral inscribed in a circle.

Foundational Standards

  • G.CO.B.8
  • G.CO.D.13

Criteria for Success


  1. Define and identify tangents, secants, and points of tangency in a circle.
  2. Prove that a line is tangent to a circle if the radius drawn to the point of tangency is perpendicular to the line. See Unit 7 Glossary for a visual.
  3. Describe that two tangent lines that intersect at a point outside the circle have congruent line segments formed from their point of intersection to their point of tangency. See Unit 7 Glossary for a visual.
  4. Describe how knowing the relationship between tangents and radii of circles can help to solve problems involving inscribed and circumscribed triangles.

Tips for Teachers


  • This lesson will prepare students to access G-C.4(+) standard by introducing them to tangents and secants of circles. In the next lesson, students will be constructing a tangent line. 
  • This lesson connects to Unit 1, Lesson 15 with inscribed and circumscribed triangles. Therefore, CFS4 is addressed mostly in the problem set guidance. 
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Anchor Problems

25-30 minutes


Problem 1

Compare the two diagrams below. How are they similar? How are they different?

Guiding Questions

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References

GeoGebra Geometry - 8.8 AP1

Geometry - 8.8 AP1 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, 11:31 a.m..

Problem 2

Below is circle $$A$$ and circle $$C$$$$\overline{CG}$$ is a diameter of circle $$A$$.$$\overleftrightarrow{EG}$$ is tangent to circle $$C$$ at point $$E$$.  $$\overleftrightarrow{FG}$$ is tangent to circle $$C$$ at point $$F$$.

  1. What are the angle measures between the tangent lines and the radii of circle $$C$$ with endpoints of $$E$$ and $$F$$?
  2. What is the relationship between $$\overline{EG}$$ and $$\overline{FG}$$?

Guiding Questions

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References

GeoGebra Geometry - 8.8 AP2

Geometry - 8.8 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, 11:33 a.m..

Modified by Fishtank Learning, Inc.
Illustrative Mathematics Tangent to a Circle from a Point

Tangent to a Circle from a Point, accessed on June 13, 2017, 11:52 a.m., is licensed by Illustrative Mathematics under either the CC BY 4.0 or CC BY-NC-SA 4.0. For further information, contact Illustrative Mathematics.

Modified by Fishtank Learning, Inc.

Target Task

5-10 minutes


The radius of circle $$A$$ shown below has a value of 2.5 cm.

What is the length of $$\overline{AD}$$?

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

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

Lesson 9
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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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