Resources for Circle Geometry Standards

Definitions and Circle Vocabulary
As an introduction to the concepts in geometry first we’ll establish a vocabulary for the key aspects of circles.
Follow along with these definitions in geogebra using the link below.
Visualizing the definitions

Radius is the measure from the center of a circle to the outside edge.
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Diameter is the measure across a circle through its center.
The diameter of a circle is twice its radius.
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Circumference is the distance around a circle.

external image circumference.png

Unrolling a circle.
Circumference can be measured in many ways.

Pi or π is the measure of the circumference of a circle with a diameter of one.
It can also be understood as the circumference divided by the diameter of a circle.
external image pi.gif
More on what Pi is
Even more on Pi

Arc is a section of the circumference of a circle.
external image arc-of-a-circle.png

Sector is a slice of a circle.
external image images?q=tbn:ANd9GcS3dUf8hyRYNFIAoattFmoyHSylA_SJWsS_tjacoSqyQjyOvhNdkQ
More on sectors

Chord is a line that links two points on a circle

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Secant is a line that intersects a circle twice.
external image secant.png

Tangent is a line that intersects a circle at exactly one point.
external image ch10no17.gif

Central Angle is an angle with endpoints located on a circle's circumference and vertex is located at the circle's center.
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Inscribed Angle is an angle made from points sitting on the circle's circumference
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Circumscribed Angle is an angle whose rays are tangent to the circle
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From these simple definitions we can build a much more detailed understanding of the geometry of circles.
Together these definitions can help us understand some useful theorems related to circles.

Angle Inscribed in a Semicircle

Theorem 1: If an angle is inscribed in a semicircle, then the angle’s measure is one half the measure of the corresponding central angle

*If you need to edit proof:
Theorem 2: If an angle is inscribed in a semicircle, then the angle is a right angle.

*if need to edit proof:

Angle at the Center Theorem:
If an angles is inscribed in a circle, then the angle’s measure is one half the measure of the corresponding central angle.

*If need to edit proof:


The angle formed by a tangent line of a circle and the radius (or diameter) of the circle to the tangent point is always 90o.
This means that if an inscribed or circumscribed triangle has the tangent line and radius or diameter as sides of the triangle, then Pythagorean rules and Pythagorean triples can be used to help solve problems of side length.

If two chords intersect inside a circle, then the product of the lengths of the segments of one chord is equal to the product of the lengths of the segments of the other chord.
Looking at the example here, this means line segment a times line segment b is equal to line segment c times line segment d:
chord intersection.PNG
chord intersection.PNG

Two tangent lines to a circle that intersect at a single point outside the circle are equal in length.
tangent intersect.PNG
tangent intersect.PNG

By extension, if a triangle is created by drawing a chord with endpoints at H and I, the resulting triangle is isosceles, and the angles at H and I are equal.
tangent intersect with chord.png
tangent intersect with chord.png

*simple practice questions using tangent rules (tangent to a circle, and two tangents intersecting at a point outside the circle)

*more challenging practice questions using tangent rules

*challenging problems involving chords, tangents, and secants

*nice applet that lets you manipulate the length of the intersecting chords and shows that the product of the line segments produced by their intersection remains equal

Below are two manipulations that can be used to help visualize the intersecting chord theorem and the inscribed angle theorem, as well as other characteristics of circles. They were made so teachers could write-up their own lesson plan using the manipulations and tailor them to their students' needs.

Inscribed Angle Theorem Manipulation:

Intersecting Chord Theorem Manipulation:

Source and a very useful site for circles and Geometry in general: