IntroductionCircles have several special properties, conjectures, postulates, and theorems associated with them. This lesson focuses on the relationship between chords and the angles and arcs they create.

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3.1.2: Chord Central Angles Conjecture

Key Concepts• Chords are segments

whose endpoints lie on the circumference of a circle.

• Three chords are shown on the circle to the right.

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued• Congruent chords

of a circle create one pair of congruent central angles.

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued• When the sides of

the central angles create diameters of the circle, vertical angles are formed. This creates two pairs of congruent central angles.

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued• Congruent chords also intercept congruent arcs. • An intercepted arc is an arc whose endpoints

intersect the sides of an inscribed angle and whose other points are in the interior of the angle.

• Remember that the measure of an arc is the same as the measure of its central angle.

• Also, recall that central angles are twice the measure of their inscribed angles.

• Central angles of two different triangles are congruent if their chords and circles are congruent.

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued• In the circle below, chords and are congruent

chords of

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued• When the radii are

constructed such that each endpoint of the chord connects to the center of the circle, four central angles are created, as well as two congruent isosceles triangles by the SSS Congruence Postulate.

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued• Since the triangles are

congruent and both triangles include two central angles that are the vertex angles of the isosceles triangles, those central angles are also congruent because Corresponding Parts of Congruent Triangles are Congruent (CPCTC).

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3.1.2: Chord Central Angles Conjecture

Key Concepts, continued

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3.1.2: Chord Central Angles Conjecture

• The measure of the arcs intercepted by the chords is congruent to the measure of the central angle because arc measures are determined by their central angle.

Common Errors/Misconceptions• mistakenly believing that central angles of different

circles will be congruent if their chords are congruent

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3.1.2: Chord Central Angles Conjecture

Guided PracticeExample 1

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3.1.2: Chord Central Angles Conjecture

Guided Practice: Example 1, continued1. Find the measure of

The measure of ∠BAC is equal to the measure of

because central angles are congruent to their

intercepted arc; therefore, the measure of is also 57°.

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3.1.2: Chord Central Angles Conjecture

Guided Practice: Example 1, continued2. Find the measure of

Subtract the measure of from 360°.

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3.1.2: Chord Central Angles Conjecture

Guided Practice: Example 1, continued3. State your conclusion.

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3.1.2: Chord Central Angles Conjecture

✔

Guided Practice: Example 1, continued

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3.1.2: Chord Central Angles Conjecture

Guided PracticeExample 2 What conclusions can you make?

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3.1.2: Chord Central Angles Conjecture

Guided Practice: Example 2, continued1. What type of angles are ∠G and ∠E, and

what does that tell you about the chords?∠G and ∠E are congruent central angles of congruent circles, so and are congruent chords.

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3.1.2: Chord Central Angles Conjecture

Guided Practice: Example 2, continued2. Since the central angles are congruent,

what else do you know? The measures of the arcs intercepted by congruent chords are congruent, so minor arc HI is congruent to minor arc JK. Deductively, then, major arc IH is congruent to major arc KJ.

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3.1.2: Chord Central Angles Conjecture

✔

Guided Practice: Example 2, continued

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3.1.2: Chord Central Angles Conjecture