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**Lines that intersect Circles**

Geometry H2 (Holt 12-1) K. Santos

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Circle definition Circle: set of all points in a plane that are a given distance (radius) from a given point (center). Circle P P Radius: is a segment that connects the center of the circle to a point on the circle

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**Interior & Exterior of a circle**

Interior of a circle: set of all the points inside the circle Exterior of a circle: set of all points outside the circle

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**Lines & Segments that intersect a circle**

A G O B F E C D Chord: is a segment whose endpoints lie on a circle. Diameter: -a chord that contains the center -connects two points on the circle and passes through the center Secant: line that intersects a circle at two points

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Tangent A tangent to a circle is a line in the plane of the circle that intersects the circle in exactly one point Tangent may be a line, ray, or segment The point where a circle and a tangent intersect is the point of tangency A B

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Pairs of circles Congruent Circles: two circles that have congruent radii Concentric Circles: coplanar circles with the same center

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Tangent Circles Tangent Circles: coplanar circles that intersect at exactly one point Internally tangent externally tangent circles circles

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Common Tangent Common tangent: a line that is tangent to two circles Common external common internal tangents tangents

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Theorem If a line is tangent to a circle, then the line is perpendicular to the radius drawn to the point of tangency. O A P B Given: 𝐴𝐵 is tangent to circle O Then: 𝐴𝐵 ⟘ 𝑂𝑃

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Example 𝐸𝐷 is tangent to circle O. Radius is 5” and ED = 12” Find the length of 𝑂𝐷 . O E D

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Example Find x. 130° x

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Theorem If a line in the plane of a circle is perpendicular to a radius at its endpoint on the circle, then the line is tangent to the circle. O A P B Given: 𝐴𝐵 ⟘ 𝑂𝑃 Then: 𝐴𝐵 is tangent to circle O

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**Example—Is there a tangent line?**

Determine if there is a tangent line?

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Theorem If two segments are tangent to a circle from the same point, then the segments are congruent. A B C Given: 𝐴𝐵 and 𝐶𝐵 are tangents to the circle Then: 𝐴𝐵 ≅ 𝐶𝐵

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Example: R 𝑅𝑇 and 𝑅𝑆 are tangent to circle Q. 2n – 1 n + 3 Find RS. T S

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12-1 Tangent Lines. Definitions A tangent to a circle is a line in the plane of the circle that intersects the circle in exactly one point called the.

12-1 Tangent Lines. Definitions A tangent to a circle is a line in the plane of the circle that intersects the circle in exactly one point called the.

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