# Chapter Three. Parallel Lines and Planes

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1 Chapter Three Parallel Lines and Planes

2 Objectives A. Use the terms defined in the chapter correctly. B. Properly use and interpret the symbols for the terms and concepts in this chapter. C. Appropriately apply the postulates and theorems in this chapter. D. Identify the special pairs of angles created by lines and transversals. E. Classify polygons according to sides and angles. F. Find the measures of interior and exterior angles. G. Compare/contrast inductive & deductive reasoning. H. Use inductive reasoning to reach a conclusion.

3 HOMEWORK NOTE! In homework problems, all proofs MUST be done as two-column proofs. Although various homework problems will ask you to write a paragraph proof, you are to write all proofs as two-column proofs.

4 Section 3-1 Definitions Homework Pages 76-77: 1-39 Excluding 18, 20, 22

5 Objectives A. Use the terms parallel lines, skew lines, and parallel planes correctly. B. Distinguish between parallel and skew lines. C. Use the term transversal correctly. D. Use the terms alternate interior angles, same-side interior angles, and corresponding angles correctly. E. Distinguish between alternate interior, same-side interior, and corresponding angles. F. Apply the terms in theorems and proofs.

6 Concepts of a parallel universe Parallel lines coplanar lines that do not intersect. The symbol for parallel lines is Skew lines non-coplanar lines that do not intersect. Parallel planes planes that do not intersect. A line is parallel to a plane if they do not intersect. m X s Y n t m n s is skew to t

7 Transversing the road of geometry Transversal A line that intersects two or more coplanar lines at different points.

8 Transversals Blue line is a transversal. Blue is a transversal for purple and red Purple is transversal for blue and red Red is transversal for blue and purple None of the lines are transversals. 6

9 Angular Relationships Alternate interior angles Two angles that are between a pair of lines (interior) and on opposite sides of a transversal (alternate). Corresponding angles Two angles that are in the same position relative to a line and a transversal eg. above-left, above-right, below-left, below-right. Same side interior angles two angles that are between a pair of lines and on the same side of a transversal. So what do you think would be the definition of alternate exterior angles?

10 Transversals & Special Pairs of Angles Given lines a and b and transversal t: 1 2 a b 7 8 t Alternate Interior Angles: 3 & 6 4 & 5 Corresponding Angles: 1 & 5 2 & 6 3 & 7 4 & 8 Same Side Interior Angles: 4 & 6 3 &

11 Theorem 3-1 If two parallel planes are cut by a third plane, then the lines of intersection are parallel.

12 Sample Problems Section 3-1 Classify each pair of angles as alternate interior, same-side interior, or corresponding angles & & & Identify the transversal first! The transversal here is the horizontal line because that line intersects two other coplanar lines at different points. 1. Alternate side of transversal, inside of transversed lines. Alternate interior angles. 3. Same side of transversal, both interior to transversed lines. Same side interior angles. 5. Same side of transversal, same relative position to transversed lines. Corresponding angles.

13 Sample Problems Section 3-1 Name the two lines and the transversal that form each pair of angles & 3 9. P & PSR & PQR S P Trace the angles first! Identify the common side. The common side is the transversal. The two non-common sides are the transversed lines. 7. Line SQ is the transversal. Lines PQ and SR are the transversed lines. 9. Line PS is the transversal. Lines PQ and SR are the transversed lines. 11. Line QR is the transversal. Lines PQ and SR are the transversed lines. R 5 Q

14 Sample Problems Section 3-1 Classify each pair of angles as alternate interior, same-side interior, or corresponding angles. 13. DCH & CBJ 15. FCL & BLC 17. GCH & GLJ Identify the transversal first! Then determine angle relationships. E F G 13. Corresponding 15. Same side interior 17. Corresponding A B C D K L J H

15 Sample Problems Section Measure a pair of alternate interior angles drawn on a piece of lined notebook paper. What appears to be true? 21. Draw a large diagram showing three transversals intersecting two nonparallel lines l and n. Number three pairs of same-side interior angles on the same side of the transversals. a. Find m 1 + m 2. b. Find m 3 + m 4. c. Predict the value of m 5 + m 6. d. What do you conclude?

16 Sample Problems Section Name five lines that appear to be parallel to AG. 25. Name four lines that appear to be skew to AB. 27. Name four planes parallel to FL. F E A D B C G L K J H I

17 Sample Problems Section 3-1 Complete each statement with always, sometimes or never. 31. Three lines intersecting in one point are sometimes coplanar._ 33. Two lines parallel to a third line are always parallel to each other. 35. Two lines perpendicular to a third line are sometimes perpendicular to each other. 37. Two planes parallel to the same plane are always parallel to each other. 39. Two lines parallel to the same plane are sometimes parallel to each other.

18 Section 3-2 Properties of Parallel Lines Homework Pages 80-82: 1-22

19 Objectives A. Use the postulate relating to parallel lines and transversals correctly. B. Apply the postulate to prove theorems relating to parallel lines. C. Use theorems of parallel lines correctly. D. Solve systems of linear equations using algebraic methods.

20 Algebra Review Solving Systems of Linear Equations From this section forward, you will need to solve systems of linear equations. This is something you learned in algebra. If you do not remember how to solve these types of problems, you will need to review the concept. A system of linear equations might look like: X + Y = 12 2X Y = 9 You can solve using isolate and substitute: X = 12 Y 2(12 Y) Y = Y Y = Y = 9 15 = 3Y Y = 5 X + 5 = 12 X = 7 You can solve using linear elimination: X + Y = 12 + (2X Y = 9) X + 0Y = 21 X = Y = 12 Y = 5 Either method produces (7, 5).

21 A New Symbol for Diagrams To denote PARALLEL LINES, use arrowheads along the pair of parallel lines. A line with a single internal arrowhead is parallel to another line with a single internal arrowhead. If multiple sets of parallel lines need to be indicated, use multiple arrowheads. The concept is similar to the one you use for denoting congruent segments, except arrowheads are used instead of slashes. l l n n

22 Postulate 10 If two parallel lines are cut by a transversal, then corresponding angles are congruent.

23 Theorem 3-2 If two parallel lines are cut by a transversal, then alternate interior angles are congruent.

24 Theorem 3-3 If two parallel lines are cut by a transversal, then same side interior angles are supplementary. 2 1 m 1 + m 2 = 180

25 Theorem 3-4 If a transversal is perpendicular to one of two parallel lines, then it is perpendicular to the other one also.

26 Sample Problems Section If a b, name all of the angles that must be congruent to 1. If you have parallel lines cut by a transversal, are corresponding angles congruent? Yes, so which angles are corresponding to 1? 3 and 9. Congruent? 3 If you have parallel lines cut by a transversal, are alternate interior angles congruent? Yes, so which angles are alternate interior to 1? NONE! If you have parallel lines cut by a transversal, are same side interior angles congruent? NO! Are there any others that might be congruent to 1 if a b? 6. Why? 8. Why? a 1 2 Transversal! 3 4 c b d

27 Assume that a b and c d. Sample Problems Section Name all the angles congruent to 2. a b c d

28 Sample Problems Section 3-2 Assume that a b and c d. 5. If m 13 = 110, then m 15 =? and m 3 =? a b c d

29 Sample Problems Section 3-2 Find the values of x and y x y x (3y+6) x (6y - 6)

30 13. Given: t l; l n Prove: t n Sample Problems Section l 1. t l 1.? 1a. 1 is a right angle 1a.? 2. m 1 = 90 2.? l n ? 3. Given 3a. 3a.? 4. m 2 = m 1 4.? m 2 = 90 5.? 5. Substitution 5a. 2 is a right angle 5a.? 6. t n 6.? t Given. Definition of perpendicular lines. Definition of a right angle. If 2 lines are cut by a transversal, then corresponding angles are congruent. Definition of congruence. Definition of a right angle. Definition of perpendicular lines. 2 n

31 Sample Problems Section Find the values of x, y and z. 110 x (z + 32) (5y + 10) 17. Given: m D = 116; AK bisects DAB; AB CD a. Find measures of DAB, KAB, and DKA. b. Is there enough information for you to conclude that D and C are supplementary, or is more information needed? D K C A B

32 Sample Problems Section Find the values of x and y. 70 (4x - 2y) (4x + 2y) 50 (4x - 2y) + 70 = 180 Why? (4x + 2y) + 50 = 180 Why? (4x - 2y) = 110 Why? (4x + 2y) = 130 Why? (8x + 0y) = 240 Why? x = 30 Why? (4(30 ) 2y) = 110 Why? (2y) = 10 Why? y = 5 Why?

33 21. Given: k l Sample Problems Section 3-2 Prove: 1 is supplementary to t 6 5 l k 23. Draw a four-sided figure ABCD with AB CD and AD BC a. Prove that A C b. Is B D

34 Section 3-3 Proving Lines Parallel Homework Pages 87-88: 1-29 Excluding 20

35 Objectives A. Use the postulate relating to determining if lines are parallel lines correctly. B. Apply the postulate to prove lines are parallel. C. Use the proof of parallel lines theorems correctly. D. Use the theorem of the existence of a parallel line through a point correctly. E. Use the theorem of the existence of a perpendicular line through a point correctly.

36 Are the Lines Parallel? Thus far we have used postulates and theorems that REQUIRE lines to be parallel. The hypothesis statement in each of these postulates and theorems requires parallel lines to exist. Based on the fact that parallel lines exist (and a transversal also exists) we can prove relationships between alternate interior, same side interior, and corresponding angles. But how do we prove that such parallel lines exist?

37 Postulate 11 If two lines are cut by a transversal and corresponding angles are congruent, then the lines are parallel.

38 Postulate or Theorem? Note that both Postulate 10, which requires parallel lines and a transversal to conclude the relationship of CORRESPONDING angles, and Postulate 11, which requires the existence of relationship between CORRESPONDING angles to conclude that lines are parallel, both deal with CORRESPONDING angles! Theorems give us the relationships between parallel lines with a transversal, same side interior angles, and alternate interior angles. Further, what is the relationship between Postulate 10 and Postulate 11? Corresponding angles are congruent if and only if they are formed by two parallel lines cut by a transversal.

39 Theorem 3-5 If two lines are cut by a transversal and alternate interior angles are congruent, then the lines are parallel.

40 Theorem 3-6 If two lines are cut by a transversal and same side interior angles are supplementary, then the lines are parallel. 2 1 m 1 + m 2 = 180

41 Theorem 3-7 In a plane if two lines are perpendicular to the same line, then those lines are parallel.

42 Theorem 3-8 Through a point outside a line, there is exactly one line parallel to the given line.

43 Theorem 3-9 Through a point outside a line, there is exactly one line perpendicular to the given line.

44 Theorem 3-10 If two lines are parallel to a third line, then those lines are parallel to each other.

45 Methods to Prove Lines Parallel Show that a pair of corresponding angles are congruent (Postulate 11). Show that a pair of alternate interior angles are congruent (Theorem 3-5). Show that a pair of same-side interior angles are supplementary (Theorem 3-6). In a plane show that both lines are perpendicular to a third line (Theorem 3-7). Show that both lines are parallel to a third line (Theorem 3-10). Note that Postulate 11 and Theorems 3-5, 3-6, and 3-7 show the existence of parallel lines by relating angles.

46 Sample Problems Section 3-3 Use the information to decide which segments, if any, are parallel. If no segments are parallel, then write none m 1 = m 8 = m 2 = m 5 7. m 1 = m 4 = 90 Draw the diagram and highlight the angles! A F 11 E B C D 1. AB FC by alternate interior angles. 3. AB FC by 2 lines tosame transversal. 5. None! 7. None!

47 Sample Problems Section 3-3 Use the information to decide which segments, if any, are parallel. If no segments are parallel, then write none. 9. m 8 + m 5 + m 6 = m 5 + m 6 = m 9 + m & 3 are comp and m 1 = m 7 = m 3 = m 10 A F 11 E B C D

48 Sample Problems Section Given: 2 1 t Prove: l n 1 3 l 2 n ? ? ? 4. l n 4.? Given Vertical angles are congruent. Transitive property of congruence. If two lines are cut by a transversal and corresponding angle are congruent, then the transversed lines are parallel.

49 Sample Problems Section Find the values of x and y that make the red and blue lines parallel. 3x 105 2y x In order for the red lines to be parallel, what must be true? 3x 105 x 35 In order for the blue lines to be parallel, what must be true? y x y 35 2y 180 2y 40 y

50 Sample Problems Section Given: 3 6. What can you prove about the other angles? Q 1 P 3 2 T S R If 3 6, can you say any lines are parallel? PQ RS If those lines are parallel, can you say any other angles are congruent? 1 4 Any other angles you KNOW to be congruent? 2 5

51 Sample Problems Section Copy what is shown for Theorem 3-7 on p 84. Then write a twocolumn proof. Theorem 3-7: In a plane, 2 lines perpendicular to the same line are parallel. t k Given : k t; n t 1 Prove: k n n 2 1. k t; n t 1. Given 2. 1and 2 are right angles. 2. Definition of perpendicular lines 3. m 1 90 and m Definition of right angles 4. m 1 m Substitution property of equality 5. Definition of congruence 6. k n 6. If 2 lines are cut by a transversal and corresponding angles are congruent, then the transversed lines are parallel.

52 Sample Problems Section Given: BE DA; CD DA Prove: 2 1 C B E D A

53 Sample Problems Section Find the measure of RST. R 40 S 70 T

54 Section 3-4 Angles of a Triangle Homework Pages 97-99: 1-30

55 Objectives A. Classify triangles according to sides and angles. B. State and apply the theorems and corollaries about the sum of the measures of the angles of a triangle. C. State and apply the theorems about the measure of an exterior angle of a triangle. D. Use the definitions relating to triangles correctly.

56 The Birth of a Triangle Triangle A figure formed by three segments joining three non-collinear points. Sides of a polygon The segments that make up a polygon. Vertex The intersection of two consecutive sides of a polygon. A Vertex Vertex B Side C Vertex

57 Other Definitions Auxiliary line A line added to a diagram to help solve a problem or write a proof. Corollary A statement proven easily by applying a theorem; a baby theorem.

58 Triangles by Sides Equilateral Triangle Scalene Triangle Isosceles Triangle Equilateral triangle A triangle in which all of the sides are congruent. Isosceles triangle A triangle in which at least two sides are congruent. Scalene triangle A triangle in which no sides are congruent. 4 6

59 Triangles by Angles Acute triangle: A triangle with three acute angles. Equiangular triangle: A triangle in which all of the angles are congruent. Right triangle: A triangle with one right angle. Obtuse triangle: A triangle with one obtuse angle. Obtuse Triangle

60 Exterior & Remote Interior Angles Exterior angle An angle formed by extending one side of a polygon beyond the vertex. Remote interior angle Two angles inside of a Exterior triangle that are not adjacent to a specific exterior angle. Remote Interiors Exterior Exterior 4 5

61 Theorem 3-11 If a polygon is a triangle, then the sum of its angles is m 1 + m 2 + m 3 = 180

62 Corollary 1 Theorem 3-11 If two angles of one triangle are congruent to two angles of another triangle, then the third angles are congruent.

63 Corollary 2 Theorem 3-11 If a triangle is equiangular, then each angle measures

64 Corollary 3 Theorem 3-11 If a polygon is a triangle, then there can be at most one right or obtuse angle.

65 Corollary 4 Theorem 3-11 If a polygon is a right triangle, then the acute angles are complementary. 1 2 m 1 + m 2 = 90

66 Theorem 3-12 If an angle is exterior to a triangle, then its measure equals the sum of the measures of the two remote interior angles m 1 + m 2 = m 3

67 Sample Problems Section 3-4 Draw a triangle that satisfies the conditions stated. If no triangle can satisfy the conditions, write not possible. 1.a. an acute isosceles triangle b. a right isosceles triangle c. an obtuse isosceles triangle. 3. a triangle with two acute exterior angles

68 Complete. Sample Problems Section m 6 + m 7 + m 8 =? 7. If m 6 = 55 and m 10 = 150, then m 8 =? Why? Theorem 3-11: If a polygon is a triangle, then the sum of its angles is Why? Theorem 3-12: If an angle is exterior to a triangle, then its measure equals the sum of the measures of the two remote interior angles

69 Complete. Sample Problems Section If m 8 = 4x, m 7 = 30, and m 9 = 6x - 20, then x =? 9. 4x + 30 = (6x 20) Why? Theorem 3-12: If an angle is exterior to a triangle, then its measure equals the sum of the measures of the two remote interior angles. 4x + 30 = (6x 20) x 50 = 2x x = (6x 20) 30 10

70 Sample Section Why? x = y x Why? y = 80 Why?

71 Sample Section x x 40 y y x = 40 Why? Why? y = 50 Why? y = 50 Why? x = 40 Why?

72 Sample Problems Section The lengths of the sides of a triangle are 4n, 2n + 10, and 7n 15. Is there a value of n that make the triangle equilateral?

73 Sample Problems Section The largest two angles of a triangle are two and three times as large as the smallest angle. Find all three measures. 21. In ABC, m A = 60 and m B < 60. What can you say about m C? 23. Given: AB BC; BD AC a. If m C = 22, find m ABD b. If m C = 23, find m ABD c. Explain why m ABD always equals m C. C B D A

74 25. Given: ABD AED Prove: C F Sample Problems Section 3-4 C B D A E F

75 Sample Problems Section Prove Theorem 3-11 by using the diagram below. 2 D

76 29. Find the values of x and y. Sample Problems Section 3-4 (2x + y) 125

77 Sample Problems Section Given AB BF; HD BF; GF BF; A G Which numbered angles must be congruent? H 3 4 B C D E F 6 A G

78 Section 3-5 Angles of a Polygon Homework Pages : 1-22 Excluding 14

79 Objectives A. Use the term polygon correctly. B. Recognize and name convex, concave, and regular polygons. C. Find the measures of interior and exterior angles of convex polygons. D. Understand and use the theorems relating to measures of interior and exterior angles of polygons correctly. E. Use the term diagonal correctly and apply it to problems and diagrams.

80 Polygon Poly Wants A poly- from Latin polys meaning many -gon from Latin gonum meaning figure of angles. Polygon is a figure with many angles polygon: a figure formed by coplanar segments such that: each segment intersects exactly two other segments, one at each endpoint no two segments with a common endpoint are collinear Diagonal A diagonal of a polygon is a segment that joins two non-consecutive vertices.

81 Polygons and Diagonals Polygons Not Polygons Diagonals

82 Classifications of Polygons Concave Polygon (non-convex) A polygon such that at least one line containing the side of a polygon contains a point in the interior of the polygon. Convex Polygon A polygon such that no line containing a side of the polygon contains a point in the interior of the polygon. Regular polygon Any polygon that is both equilateral and equiangular.

83 Concave & Convex Polygons Concave Convex 3

84 Polygons Triangle Quadrilateral Pentagon Hexagon Octagon 5

85 Names for Polygons triangle: 3 sides quadrilateral: 4 sides pentagon: 5 sides hexagon: 6 sides octagon: 8 sides nonagon: 9 sides decagon: 10 sides dodecagon: 12 sides n-gon: n sides

86 Theorem 3-13 If a polygon is convex with n sides, then the sum of the measures of the interior angles is equal to (n - 2)180. (3-2)180 (4-2)180 (5-2)180 (6-2)180 (8-2)180

87 Theorem 3-14 If a polygon is convex with n sides, then the sum of the measures of the exterior angles, one angle at each vertex, is 360. = 360 = 360 = 360 = 360 = 360

88 Sample Problems Section 3-5 For each polygon find (a) the interior angle sum and (b) the exterior angle sum. 1. quadrilateral 3. hexagon 5. decagon 9. A baseball diamond s home plate has three right angles. The other two angles are congruent. Find their measure. 11. The face of a honeycomb consists of interlocking regular hexagons. What is the measure of each angle of these hexagons?

89 Sample Problems Section 3-5 Sketch the polygon described. If no such polygon exists, write not possible. 13. A quadrilateral that is equilateral but not equiangular. 15. A regular polygon, one of whose angles has a measure of The measure of each interior angle of a regular polygon is eleven times that of an exterior angle. How many sides does the polygon have? 19. Make a sketch showing how to tile a floor using both squares and regular octagons. 21. In quadrilateral ABCD, m A = x, m B = 2x, m C = 3x and m D = 4x. Find the value of x and then state which pair of sides of ABCD must be parallel.

90 Sample Problems Section ABCDEFGHIJ is a regular decagon. If sides AB and CD are extended to meet at K, find the measure of K. 25. The sum of the measures of the interior angles of a polygon is known to be between 2100 and How many sides does the polygon have?

91 Section 3-6 Inductive Reasoning Homework Pages : 1-24

92 Objectives A. Explain the difference between inductive and deductive reasoning. B. Properly apply deductive and inductive reasoning to problems. C. Understand the importance of inductive reasoning.

93 Deductive vs. Inductive Reasoning Deductive Reasoning conclusion based on accepted statements such as definitions, postulates, properties, theorems, corollaries & given information conclusion must be true if the hypothesis is true Inductive Reasoning conclusion based on several past observations such as those observed through experimentation conclusion is probably true, but not necessarily true

94 Sample Problems Section 3-6 Look for a pattern and predict the next two terms in the sequence. 1. 1, 4, 16, 64, ,,,, , 3, 5, 8, 12, 7. 40, 39, 36, 31, 24, , 20, 10, 100,.50,...

95 Sample Problems Section 3-6 Accept the two statements as given information. State a conclusion based on deductive reasoning. If no conclusion can be reached, then write none. 11. Valerie is older than Greg. Dan is older than Greg. 13. Polygon G has more than six sides. Polygon K has more than six sides.

96 Sample Problems Section 3-6 For each exercise write the equation that you think should come next x = x = x = = = =

97 Sample Problems Section 3-6 Decide whether each statement is true or false. If it is false show a counterexample. If it is true, draw and label a diagram you could use in a proof. 19. If a triangle has two congruent angles, then the sides opposite those angles are congruent. 21. All diagonals of a regular pentagon are congruent. 23. If the diagonals of a quadrilateral are congruent and also perpendicular, then the quadrilateral is a regular quadrilateral. 25. The diagonals of an equilateral quadrilateral are perpendicular.

98 Chapter Three Parallel Lines and Planes Review Homework pages : 2-14 evens

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