Circles

Lesson 1

Math

Unit 7

10th Grade

Lesson 1 of 14

Objective


Derive the equation of a circle using the Pythagorean Theorem where the center of the circle is at the origin.

Common Core Standards


Core Standards

  • G.GPE.A.1 — Derive the equation of a circle of given center and radius using the Pythagorean Theorem; complete the square to find the center and radius of a circle given by an equation.
  • G.GPE.B.4 — Use coordinates to prove simple geometric theorems algebraically. For example, prove or disprove that a figure defined by four given points in the coordinate plane is a rectangle; prove or disprove that the point (1, √3) lies on the circle centered at the origin and containing the point (0, 2).

Foundational Standards

  • G.GPE.B.7
  • 8.G.B.7
  • 8.G.B.8

Criteria for Success


  1. Describe the line segment whose endpoints are the center of the circle and a point on the circle as the hypotenuse of a right triangle formed inside the circle. 
  2. Derive the equation of a circle centered at the origin as $$x^2+y^2=r^2$$ using repeated reasoning. 
  3. Understand and describe the relationship between the equation of a circle at the origin and the Pythagorean Theorem. 
  4. Determine the equation of a circle given the center of the circle at the origin and one point on the circle.
  5. Given the equation of a circle centered at the origin, identify the radius and points that lie on the circle. 

Tips for Teachers


  • Many sources on this topic combine writing equations of circles with a center at the origin and writing equations of circles with a center not at the origin. In this unit, these two topics are separated to allow students to have the opportunity to practice the algebraic skills associated with each component. 
  • Students may need to review algebra work with solving equations using square roots and substituting values into an equation with quadratic terms before they can fully access this lesson. 
  • ADDITIONAL REVIEW: Because this lesson has a lot of conceptual components, in order to prepare for Lesson 3, teachers should take the opportunity through the independent practice to review completing the square to write quadratic equations in vertex form.
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Anchor Problems

25-30 minutes


Problem 1

Write an equation that models each point’s distance from the center of the circle, point $$A$$.

Guiding Questions

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References

GeoGebra Geometry-8.1 AP1

Geometry-8.1 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 7, 2017, 9:05 a.m..

Problem 2

A circle has a center at the origin and contains the point ($$2, \space \sqrt{5}$$).

  1. Write an equation of the circle.
  2. Identify the radius of this circle.
  3. Name two other points that lie on this circle. 

Guiding Questions

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Target Task

5-10 minutes


The circle shown below has a center at the origin.

If a point ($$3, \space y$$) lies on the circle, what is the value of $$y$$?

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 must identify whether an equation forms a circle. For example, "Joe says that the equation $$x^2+y^2=-4$$ forms a circle with radius $$4$$. Why is Joe not correct?"
  • Include problems where various information is given and ask students to find the radius, one point, or the equation of the circle. Again, only use circles centered at the origin. 
  • EXTENSION: Give a diagram similar to Math Mistakes’ “Circumference and Right Triangles” and ask students to write the equation of the circle. 

Next

Given a circle with a center translated from the origin, write the equation of the circle and describe its features.

Lesson 2
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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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