# Lesson 10.1 Circles. Definition: The set of all points in a plane that are a given distance from a given point in the plane. The given point is the CENTER.

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Lesson 10.1 Circles

Definition: The set of all points in a plane that are a given distance from a given point in the plane. The given point is the CENTER of the circle. A segment that joins the center to a point on the circle is called a radius. Two circles are congruent if they have congruent radii.

Concentric Circles: Two or more coplanar circles with the same center.

A point is inside (in the interior of) a circle if its distance from the center is less than the radius. interior O A Point O and A are in the interior of Circle O.

A point is outside (in the exterior of) a circle if its distance from the center is greater than the radius. A W Point W is in the exterior of Circle A. A point is on a circle if its distance from the center is equal to the radius. S Point S is on Circle A.

Chords and Diameters: Points on a circle can be connected by segments called chords. A chord of a circle is a segment joining any two points on the circle. A diameter of a circle is a chord that passes through the center of the circle. The longest chord of a circle is the diameter. chord diameter

Formulas to know! Circumference: C = 2 π r or C = π d Area: A = π r 2 Area: A = π r 2

Radius-Chord Relationships OP is the distance from O to chord AB. The distance from the center of a circle to a chord is the measure of the perpendicular segment from the center to the chord.

Theorem 74 If a radius is perpendicular to a chord, then it bisects the chord.

Theorem 75 If a radius of a circle bisects a chord that is not a diameter, then it is perpendicular to that chord.

Theorem 76 The perpendicular bisector of a chord passes through the center of the circle.

1. Circle Q, PR  ST 2.PR bisects ST. 3.PR is  bisector of ST. 4.PS  PT 1.Given 2.If a radius is  to a chord, it bisects the chord. (QR is part of a radius.) 3.Combination of steps 1 & 2. 4.If a point is on the  bisector of a segment, it is equidistant from the endpoints.

The radius of Circle O is 13 mm. The length of chord PQ is 10 mm. Find the distance from chord PQ to center, O. 1.Draw OR perpendicular to PQ. 2.Draw radius OP to complete a right Δ. 3.Since a radius perpendicular to a chord bisects the chord, PR = ½ PQ = ½ (10) = 5. 4.By the Pythagorean Theorem, x 2 + 5 2 = 13 2 5.The distance from chord PQ to center O is 12 mm.

1.ΔABC is isosceles (AB  AC) 2.Circles P & Q, BC ║ PQ 3.  ABC   P,  ACB   Q 4.  ABC   ACB 5.  P   Q 6.AP  AQ 7.PB  CQ 8.Circle P  Circle Q 1.Given 2.Given 3.║ Lines means corresponding  s . 4.. 5.Transitive Property 6.. 7.Subtraction (1 from 6) 8.Circles with  radii are .

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