210.1 Tangents to CirclesCircle: the set of all points in a plane that are equidistant from a given point.Center: the given point.Radius: a segment whose endpoints are the center of the circle and a point on the circle.
3Vocabulary Chord: a segment whose endpoints are points on the circle. Diameter: a chord that passes through the center of the circle.Secant: a line that intersects a circle in two points.Tangent: a line in the plane of a circle that intersects the circle in exactly one point.
4More VocabularyCongruent Circles: two circles that have the same radius.Concentric Circles: two circles that share the same center.Tangent Circles: two circles that intersect in one point.
5Tangent TheoremsIf a line is tangent to a circle, then it is perpendicular to the radius drawn to the point of tangency.PQ
6Tangent TheoremsIn a plane, if a line is perpendicular to a radius of a circle at its endpoint on the circle, then the line is tangent to the circle.PQ
7Tangent TheoremsIf two segments from the same exterior point are tangent to a circle then they are congruent.QP
8Examples Find an example for each term: Center Chord Diameter Radius Point of TangencyCommon external tangentCommon internal tangentSecant
9Examples The diameter is given. Find the radius. d=15cm d=6.7in d=3ft
10Examples The radius is given. Find the diameter. r = 26in r = 62ft r = 4.4cm
11ExamplesTell whether AB is tangent to C.A145B15C
12ExamplesTell whether AB is tangent to C.A12C168B
13ExamplesAB and AD are tangent to C. Find x.D2x + 7A5x - 8B
1410.2 Arcs and ChordsAn angle whose vertex is the center of a circle is a central angle.If the measure of a central angle is less than 180 , then A, B and the points in the interior of APB form aminor arc.Likewise, if it is greaterthan 180, if forms a majorarc.
15Arcs and ChordsIf the endpoints of an arc are the endpoints of a diameter, then the arc is a semicircle.The measure of an arc is the same as the measure of its central angle.
19TheoremsIn the same circle, or in congruent circles, congruent chords have congruent arcs and congruent arcs have congruent chords.In the same circle, or in congruent circles, two chords are congruent iff they are equidistant from the center.
20Examples Determine whether the arc is minor, major or a semicircle. PQ SUQTTUPPUQ
21Examples KN and JL are diameters. Find the indicated measures. mKL mMN mMKNmJML
22ExamplesFind the value of x. Then find the measure of the red arc.
2410.3 Inscribed AnglesAn inscribed angle is an angle whose vertex is on a circle and whose sides contain chords of the circle.The arc that lies in the interior of an inscribed angle and has endpoints on the angle is called the intercepted arc.
25Measure of an Inscribed Angle If an angle is inscribed in a circle, then its measure is half the measure of its intercepted arc.mADB = ½ mAB
26Find the measure of the blue arc or angle mQTS =m NMP =
27TheoremIf two inscribed angles if a circle intercept the same arc, then the angles are congruent.
28Polygons and circlesIf all of the vertices of a polygon lie on a circle, the polygon is inscribed in the circle, and the circle is circumscribed about the polygon.
29TheoremsIf a right triangle is inscribed in a circle, then the hypontenuse is a diameter of the circle.B is a right angle iff AC is a diameter of the circle.
30TheoremsA quadrilateral can be inscribed in a circle iff its opposite angles are supplementary.D, E, F, and G lie on some circle, C, if and only ifm D + m F = 180° and m E + m G = 180°
3310.4 Other Angle Relationships in Circles If a tangent and a chord intersect at a point on a circle, then the measure of each angle formed is one half the measure of its intercepted arc.m 1 = ½ mABm 2 = ½ mBCA
34ExamplesLine m is tangent to the circle. Find the measure of the red angle or arc.
35ExamplesBC is tangent to the circle. Find m CBD.
36TheoremsIf two chords intersect in the interior of a circle, then the measure of each angle is one half the sum of the measures of the arcs intercepted by the angle and its vertical angle.x = ½ (mPS + mRQ)x = ½ ( )x = 140
37Theorems m 1 = ½ (mBC – mAC) m 2 = ½ (mPQR – mPR) m 3 = ½ (mXY – mWZ)
3910.5 Segment Lengths in Circles When 2 chords intersect inside of a circle, each chord is divided into 2 segments, called segments of a chord.When this happens, 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.
4710.6 Equations of CirclesYou can write the equation of a circle in a coordinate plane if you know its radius and the coordinates of its center.Suppose the radius is r and its center is at ( h, k)(x – h)2 + (y – k)2 = r2(standard equation of a circle)
48ExamplesIf the circle has a radius of 7.1 and a center at ( -4, 0), write the equation of the circle.(x – h)2 + (y – k)2 = r2(x – -4)2 + (y – 0)2 = 7.12(x + 4)2 + y2 = 50.41
49ExamplesThe point (1, 2) is on a circle whose center is (5, -1). Write the standard equation of the circle.Find the radius. (Use the distance formula).(x – 5)2 + (y – -1)2 = 52(x – 5)2 + (y +1)2 = 25
50Graphing a CircleThe equation of the circle is: (x + 2)2 + (y – 3)2 = 9Rewrite the equation to find the center and the radius.(x – (-2))2 + (y – 3)2 = 32The center is (-2, 3) and the radius is 3.
51Graphing a CircleThe center is (-2, 3) and the radius is 3.