The Equation of a Circle

Great geometry lesson

Easing the Hurry Syndrome

Expressing Geometric Properties with Equations

G-GPE.A Translate between the geometric description and the equation for a conic section

  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.

How do you provide an opportunity for your students to make sense of the equation of a circle in the coordinate plane? We recently use the Geometry Nspired activity Exploring the Equation of a Circle.

Students practiced look for and express regularity in repeated reasoning. What stays the same? What changes?

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It’s a right triangle.

The hypotenuse is always 5.

The legs change.

What else do you notice? What has to be true for these objects?

The Pythagorean Theorem works.


Leg squared plus leg squared equals five squared.

What do you notice about the legs? How can we…

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Published by

Michelle Bonds

I was an engineer for 6 years before I became a teacher. I've taught math 6th - 12th grade, computer science, and EAST (Environmental and Spatial Technologies). I also provide professional development in the areas of technology and STEM learning. I have been providing professional development since 2007 locally and nationally. I created an Engineering Club at my current high school for students interested in STEM fields and am proud to be the 2011 Student Racing Challenge National Champs. My two personal passions are NASCAR and travelling.

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