# Lesson 1: Introducing Circles

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1 IRLES N VOLUME Lesson 1: Introducing ircles ommon ore Georgia Performance Standards M9 12.G..1 M9 12.G..2 Essential Questions 1. Why are all circles similar? 2. What are the relationships among inscribed angles, radii, and chords of a circle? 3. What are the relationships among circumscribed angles, central angles, and inscribed angles? 4. What is the relationship between a tangent line and the radius of a circle? WORS TO KNOW arc central angle chord circle circumference circumscribed angle concentric circles congruent arcs part of a circle s circumference an angle with its vertex at the center of a circle a segment whose endpoints lie on the circumference of the circle the set of all points that are equidistant from a reference point, the center. The set of points forms a 2-dimensional curve that measures 360º. the distance around a circle; = 2 r or = d, for which represents circumference, r represents the circle s radius, and d represents the circle s diameter. the angle formed by two tangent lines whose vertex is outside of the circle coplanar circles that have the same center two arcs that have the same measure and are either of the same circle or of congruent circles U3-1 Lesson 1: Introducing ircles

2 diameter inscribed angle intercepted arc major arc minor arc pi ( ) radius secant line semicircle tangent line a straight line passing through the center of a circle connecting two points on the circle; equal to twice the radius an angle formed by two chords whose vertex is on the circle an arc whose endpoints intersect the sides of an inscribed angle and whose other points are in the interior of the angle part of a circle s circumference that is larger than its semicircle part of a circle s circumference that is smaller than its semicircle the ratio of circumference of a circle to the diameter; equal to approximately 3.14 the distance from the center to a point on the circle; equal to one-half the diameter a line that intersects a circle at two points an arc that is half of a circle a line that intersects a circle at exactly one point and is perpendicular to the radius of the circle U3-2 Unit 3: ircles and Volume

3 Recommended Resources Math Open Reference. entral ngle Theorem. This site describes the entral ngle Theorem and allows users to explore the relationship between inscribed angles and central angles. Math Warehouse. What Is the Tangent of a ircle? This site reviews the properties of tangent lines, and allows users to interactively explore the idea that a tangent line is perpendicular to a radius at the point of tangency. This site also provides limited practice problems, as well as solutions. RicksMath.com. Geometry and the ircle. This site defines and provides examples of relationships of circles and their properties. U3-3 Lesson 1: Introducing ircles

4 Lesson 3.1.1: Similar ircles and entral and Inscribed ngles Introduction In the third century.., Greek mathematician Euclid, often referred to as the Father of Geometry, created what is known as Euclidean geometry. He took properties of shape, size, and space and postulated their unchanging relationships that cultures before understood but had not proved to always be true. rchimedes, a fellow Greek mathematician, followed that by creating the foundations for what is now known as calculus. In addition to being responsible for determining things like the area under a curve, rchimedes is credited for coming up with a method for determining the most accurate approximation of pi,. In this lesson, you will explore and practice applying several properties of circles including proving that all circles are similar using a variation of rchimedes method. Key oncepts Pi, ( ), is the ratio of the circumference to the diameter of a circle, where the circumference is the distance around a circle, the diameter is a segment with endpoints on the circle that passes through the center of the circle, and a circle is the set of all points that are equidistant from a reference point (the center) and form a 2-dimensional curve. circle measures 360º. oncentric circles share the same center. The diagram below shows circle ( ) with diameter and radius. The radius of a circle is a segment with endpoints on the circle and at the circle s center; a radius is equal to half the diameter. radius diameter ll circles are similar and measure 360º. U3-4 Unit 3: ircles and Volume

5 portion of a circle s circumference is called an arc. The measure of a semicircle, or an arc that is equal to half of a circle, is 180º. rcs are named by their endpoints. The semicircle in the following diagram can be named. m = 180 ) part of the circle that is larger than a semicircle is called a major arc. It is common to identify a third point on the circle when naming major arcs. The major arc in the following diagram can be named. minor arc is a part of a circle that is smaller than a semicircle. U3-5 Lesson 1: Introducing ircles

6 The minor arc in the following diagram can be named. Two arcs of the same circle or of congruent circles are congruent arcs if they have the same measure. The measure of an arc is determined by the central angle. central angle of a circle is an angle with its vertex at the center of the circle and sides that are created from two radii of the circle. chord is a segment whose endpoints lie on the circumference of a circle. n inscribed angle of a circle is an angle formed by two chords whose vertex is on the circle. U3-6 Unit 3: ircles and Volume

7 n inscribed angle is half the measure of the central angle that intercepts the same arc. onversely, the measure of the central angle is twice the measure of the inscribed angle that intercepts the same arc. This is called the Inscribed ngle Theorem. Inscribed ngle Theorem The measure of an inscribed angle is half the measure of its intercepted arc s angle. Given, m = 1 m. 2 In the following diagram, is the inscribed angle and is the central angle. They both intercept the minor arc. 2x x U3-7 Lesson 1: Introducing ircles

8 orollaries to the Inscribed ngle Theorem orollary 1 orollary 2 Two inscribed angles that intercept the same arc are congruent. n angle inscribed in a semicircle is a right angle. E U3-8 Unit 3: ircles and Volume

9 Guided Practice Example 1 Prove that the measure of a central angle is twice the measure of an inscribed angle that intercepts the same arc. Given: with inscribed and central intercepting. Prove: 2m = m 1. Identify the known information. ircle with inscribed and central intercepts. Let be a diameter of the circle. y definition, and are radii of the circle. Mark them as r. Identify as x and as y. r y r x U3-9 Lesson 1: Introducing ircles

10 2. Identify what information is known about the angles of the triangle. is isosceles since both legs are the same length, r. y the Isosceles Triangle Theorem, both base angles of the triangle are congruent; therefore, = x. lso, the vertex angle of the isosceles triangle is a linear pair with y and thus m = 180 y. x r y r x y the Triangle Sum Theorem: x + x y = 180 2x y = 0 2x = y Substituting the names for x and y yields 2m = m. Therefore, the measure of a central angle is twice the measure of an inscribed angle that intercepts the same arc. U3-10 Unit 3: ircles and Volume

11 Example 2 Prove that all circles are similar using the concept of similarity transformations. 1. reate a diagram to help with the proof. raw with radius r 1. r 1 raw with radius r 2 such that r 2 > r 1. r 2 U3-11 Lesson 1: Introducing ircles

12 2. Using properties of rigid motion, translate so that it is concentric with. r 2 r 1 3. etermine the scale factor necessary to dilate so that it maps to. To determine the ratio, note that each circle has a radius that by definition is the segment from the center to a point on the circle. To map, divide r 2 by r 1. The resulting ratio is the scale factor that produces the following image. r 2 r 1 ' This scale factor, r 2, is true for all circles and a similarity r1 transformation of this sort can always be used because all circles are similar. U3-12 Unit 3: ircles and Volume

13 Example 3 car has a circular turning radius of 15.5 feet. The distance between the two front tires is 5.4 feet. To the nearest foot, how much farther does a tire on the outer edge of the turning radius travel than a tire on the inner edge? 15.5 ft 5.4 ft 1. alculate the circumference of the outer tire s turn. = 2πr Formula for the circumference of a circle = 2π ( 155. ) =31π Substitute 15.5 for the radius (r). Simplify. 2. alculate the circumference of the inside tire s turn. First, calculate the radius of the inner tire s turn. Since all tires are similar, the radius of the inner tire s turn can be calculated by subtracting the distance between the two front wheels (the distance between each circle) from the radius of the outer tire s turn = 10.1 ft = 2πr Formula for the circumference of a circle = 2π ( 101. ) =202. π Simplify. Substitute 10.1 for the radius (r). U3-13 Lesson 1: Introducing ircles

14 3. alculate the difference in the circumference of each tire s turn. Find the difference in the circumference of each tire s turn. 31π 202. π= 108. π The outer tire travels approximately 34 feet farther than the inner tire. Example 4 Find the value of each variable. a E b c F 1. Identify the inscribed angle and the intercepted arc and use the proper theorem or corollary to find the value of a. a is the inscribed angle creating the intercepting arc. The measure of the intercepted arc is 70º. Use the Inscribed ngle Theorem to find a. m E= m 2 Inscribed ngle Theorem 1 a= ( 70) 2 Substitute the given information. a = 35 Solve for a. The value of a is 35º. U3-14 Unit 3: ircles and Volume

15 2. Identify the inscribed angle and the intercepted arc and use the proper theorem or corollary to find the value of b. b is the inscribed angle creating the intercepting arc EF. The measure of the intercepted arc is 104º. Use the Inscribed ngle Theorem to find b. m = mef 2 Inscribed ngle Theorem 1 b= ( 104) 2 Substitute the given information. b = 52 Solve for b. The value of b is 52º. 3. Use the Triangle Sum Theorem and the Vertical ngles Theorem to find the value of c. Note that a and b, in addition to being the measures of inscribed angles, are interior angles of a triangle. The third interior angle is a vertical angle to c and is therefore congruent to c. Finding that third angle will yield the value of c. a + b + c = 180 Triangle Sum Theorem and Vertical ngles Theorem c = 180 Substitute the measures of a and b c = 180 ombine like terms. c = 93 Solve for c. The value of c is 93º. U3-15 Lesson 1: Introducing ircles

16 Example 5 Find the measures of and. (7x 7) (x + 14) 1. Set up an equation to solve for x. is a central angle and is an inscribed angle in. m = 2m entral ngle/inscribed ngle Theorem 7x 7 = 2(x + 14) Substitute values for and. 7x 7 = 2x + 28 istributive Property 5x = 35 Solve for x. x = 7 2. Substitute the value of x into the expression for to find the measure of the inscribed angle. m = ( x+ 14 ) m = ( 7) + 14 m =21 The measure of is 21º. U3-16 Unit 3: ircles and Volume

17 3. Find the value of the central angle,. y the Inscribed ngle Theorem, m = 2 m. m =21( 2) m =42 The measure of is 42º. U3-17 Lesson 1: Introducing ircles

18 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles Practice 3.1.1: Similar ircles and entral and Inscribed ngles Given that all circles are similar, determine the scale factor necessary to map. 1. has a radius of 144 units and has a radius of 3 units. 2. has a diameter of 50 units and has a diameter of 12 units. Use your knowledge of similar circles to complete problems 3 and circular waterfall is surrounded by a brick wall. The radius of the inner wall is 35 inches. If the bricks are 5 inches wide, what scale factor was used to determine the radius of the outer wall? 35 in 5 in 4. small car has a tire with a 17-inch diameter. truck has a tire with a 32-inch diameter. How much farther than the car does the truck have to drive for its tire to complete one revolution? U3-18 Unit 3: ircles and Volume continued

19 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles Use your knowledge of angles to complete the problems that follow. 5. Find the values of x, y, and z. x z y 6. Find the value of x and the measure of. (x + 7) (3x 5) continued U3-19 Lesson 1: Introducing ircles

20 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 7. Find the values of x and y. 15 y x 8. Find m and m. ( x + 5) 3 (x 35) continued U3-20 Unit 3: ircles and Volume

21 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 9. Find m and m. y 100 (2x + 10) 10. Find m and m U3-21 Lesson 1: Introducing ircles

22 Lesson 3.1.2: hord entral ngles onjecture Introduction ircles 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. Key oncepts hords are segments whose endpoints lie on the circumference of a circle. Three chords are shown on the circle below. ongruent chords of a circle create one pair of congruent central angles. U3-22 Unit 3: ircles and Volume

23 When the sides of the central angles create diameters of the circle, vertical angles are formed. This creates two pairs of congruent central angles. E ongruent chords also intercept congruent arcs. n 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. lso, recall that central angles are twice the measure of their inscribed angles. entral angles of two different triangles are congruent if their chords and circles are congruent. In the circle below, chords and E are congruent chords of. E U3-23 Lesson 1: Introducing ircles

24 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 ongruence Postulate. E 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 orresponding Parts of ongruent Triangles are ongruent (PT). E 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. x x x x E U3-24 Unit 3: ircles and Volume

25 Guided Practice Example 1 In, m =57. What is m? Find the measure of. m =57 The measure of is equal to the measure of because central angles are congruent to their intercepted arc; therefore, the measure of is also 57º. 2. Find the measure of. Subtract the measure of from 360º = 303 m State your conclusion. The measure of is 303º. U3-25 Lesson 1: Introducing ircles

26 Example 2 G E. What conclusions can you make? K G I E J H 1. 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 HI and JK are congruent chords. 2. 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. eductively, then, major arc IH is congruent to major arc KJ. U3-26 Unit 3: ircles and Volume

27 Example 3 Find the value of y. (13y 48) (8y 47) E 1. Set up an equation to solve for y. The angles marked are central angles created by congruent chords, and are therefore congruent. Set the measure of each angle equal to each other. 13y 48 = 8y Solve for y. 13y 48 = 8y y = 95 y = 19 entral angles created by congruent chords are congruent. U3-27 Lesson 1: Introducing ircles

28 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles Practice 3.1.2: hord entral ngles onjecture Use what you ve learned about chords and central angles to solve. 1. In, m =72. What is m? In, 315. What is m? What is the value of w? (43 3w) (w + 9) continued U3-28 Unit 3: ircles and Volume

29 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 4. What can you conclude about G and E? K G I E J H 5. G E. What is the value of a? K 3a G (6a 24) I E J H 6. Find the value of b. (5b 9) (8 2b) E continued U3-29 Lesson 1: Introducing ircles

30 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 7. Find the value of b. (8b + 1) (12 + 6b) E 8. Find the value of b. (8b + 11) (10b + 3) E 9. If the circumference of one circle is 54 millimeters and the major arc of a second circle measures 182º, what is the length of this major arc of the second circle? 10. You re sewing the seam on a circular lampshade for interior design class. The total circumference of the lampshade is 3 feet. The amount of the seam you ve sewn so far measures approximately 23º around the circumference of the lampshade. What length of lampshade still needs to be sewn? U3-30 Unit 3: ircles and Volume

31 Lesson 3.1.3: Properties of Tangents of a ircle Introduction ircles and tangent lines can be useful in many real-world applications and fields of study, such as construction, landscaping, and engineering. There are many different types of lines that touch or intersect circles. ll of these lines have unique properties and relationships to a circle. Specifically, in this lesson, we will identify what a tangent line is, explore the properties of tangent lines, prove that a line is tangent to a circle, and find the lengths of tangent lines. We will also identify and use secant lines, as well as discuss how they are different from tangent lines. Key oncepts tangent line is a line that intersects a circle at exactly one point. Tangent lines are perpendicular to the radius of the circle at the point of tangency. Point of tangency Radius Tangent line You can verify that a line is tangent to a circle by constructing a right triangle using the radius, and verifying that it is a right triangle by using the Pythagorean Theorem. The slopes of a line and a radius drawn to the possible point of tangency must be negative reciprocals in order for the line to be a tangent. If two segments are tangent to the same circle, and originate from the same exterior point, then the segments are congruent. The angle formed by two tangent lines whose vertex is outside of the circle is called the circumscribed angle. U3-31 Lesson 1: Introducing ircles

32 is a circumscribed angle. O The length of tangent is equal to the length of tangent. The angle formed by two tangents is equal to one half the positive difference of the angle s intercepted arcs. secant line is a line that intersects a circle at two points. Secant line n angle formed by a secant and a tangent is equal to the positive difference of its intercepted arcs. U3-32 Unit 3: ircles and Volume

33 Guided Practice Example 1 etermine whether is tangent to in the diagram below Identify the radius. The radius of the circle is the segment from the center of the circle to a point on the circle. is the radius of. 2. etermine the relationship between and at point in order for to be tangent to. must be perpendicular to at point in order for to be tangent to. U3-33 Lesson 1: Introducing ircles

34 3. Show that is a right angle by using the converse of the Pythagorean Theorem. The converse of the Pythagorean Theorem states that when the sum of the squares of two sides of a triangle is equal to the square of the third side of the triangle, the triangle is a right triangle. a 2 + b 2 = c 2 Pythagorean Theorem () 2 + () 2 = () 2 Substitute segment names for a, b, and c = 41 2 Substitute values for,, and = 1681 Simplify, then solve = 1681 The result is a true statement; therefore, is a right angle. 4. State your conclusion. Since the converse of the Pythagorean Theorem is true, is a right triangle; therefore, is a right angle. This makes perpendicular to ; therefore, is tangent to. U3-34 Unit 3: ircles and Volume

35 Example 2 Each side of is tangent to circle O at the points, E, and F. Find the perimeter of. 7 F O 5 E Identify the lengths of each side of the triangle. is tangent to the same circle as F and extends from the same point; therefore, the lengths are equal. = 7 units E is tangent to the same circle as and extends from the same point; therefore, the lengths are equal. E = 5 units To determine the length of E, subtract the length of E from the length of = 11 E = 11 units F is tangent to the same circle as E and extends from the same point; therefore, the lengths are equal. F = 11 units U3-35 Lesson 1: Introducing ircles

36 2. alculate the perimeter of. dd the lengths of, F,, E, E, and F to find the perimeter of the polygon = 46 units The perimeter of is 46 units. Example 3 landscaper wants to build a walkway tangent to the circular park shown in the diagram below. The other walkway pictured is a radius of the circle and has a slope of 1 on the grid. If the walkways should intersect at (4, 2) on the grid, what equation 2 can the landscaper use to graph the new walkway on the grid? y x U3-36 Unit 3: ircles and Volume

37 1. etermine the slope of the walkway. One of the properties of tangent lines states that if a line is tangent to a circle, then it is perpendicular to the radius at the point of tangency. Since the slope of the radius is 1, then the slope of the new walkway will 2 be 2 because perpendicular lines have slopes that are negative reciprocals. 2. etermine the equation that represents the walkway. Recall that the point-slope form of a line is y y = m( x x ). 1 1 Since the new walkway is tangent to the circle at (4, 2), then the point (4, 2) is on the tangent line, so it can be used to write the equation. y ( 2) = 2( x 4) y+ 2= 2( x 4) This equation can be rearranged into slope-intercept form, y= 2x 10, for easier graphing. Example 4 is tangent to at point as shown below. Find the length of as well as m U3-37 Lesson 1: Introducing ircles

38 1. Find the length of. Since is tangent to, then is right angle because a tangent and a radius form a right angle at the point of tangency. Since is a right angle, is a right triangle. Use the Pythagorean Theorem to find the length of. a 2 + b 2 = c 2 Pythagorean Theorem () 2 = 17 2 Substitute values for a, b, and c () 2 = 289 Simplify. () 2 = 225 = 15 The length of is 15 units. 2. Find m. First, determine the unknown measure of. Recall that the sum of all three angles of a triangle is 180º. is a right angle, so it is 90º. is 28º, as shown in the diagram. Set up an equation to determine the measure of m = m = 180 m = 62 Since m = 62, then m = 118 because and are a linear pair. is a central angle, and recall that the measure of a central angle is the same as its intercepted arc, so is 118º. U3-38 Unit 3: ircles and Volume

39 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles Practice 3.1.3: Properties of Tangents of a ircle Use what you have learned about tangent lines and secant lines to answer the questions. 1. and are tangent to L in the diagram below. What is the value of x? (3x 10) L (x + 6) 2. You know that PR is tangent to S in the diagram below. What must you prove to show that this is true? Q P S R continued U3-39 Lesson 1: Introducing ircles

40 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 3. is tangent to at point in the diagram below. What is the measure of? space station orbiting the Earth is sending two signals that are tangent to the Earth. If the intercepted arc of the Earth s surface that is visible to the satellite is 168º, what is the measure of the angle formed by the two signal beams? 5. Is XY tangent to Z at point X in the diagram below? Explain. X 55 Y Z continued U3-40 Unit 3: ircles and Volume

41 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 6. How many tangents can be drawn that contain a point on a circle? Explain your answer. 7. The slope of radius PQ in circle Q is 2. student wants to draw a tangent to 3 Q at point P. What will be the slope of this tangent line? 8. homeowner is building a square fence around his circular patio so that each side of the fence is tangent to the circle, as shown in the diagram below. The radius of his patio is 18 yards. What is the perimeter of his fence? continued U3-41 Lesson 1: Introducing ircles

42 PRTIE IRLES N VOLUME Lesson 1: Introducing ircles 9. You are using binoculars to look at an eagle. The sides of the binoculars seem to extend from the eagle, and are tangent to the circular portion. How far away is the eagle? Eagle (x + 51) ft (18x) ft 10. Your friend Truman argues that chords and tangents are the same thing. You disagree. What do you tell Truman to convince him that he is incorrect? U3-42 Unit 3: ircles and Volume

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