G E O M E T R Y Chapter 3: Angle Pairs, Lines and Planes

1. GEOMETRY Chapter 3: Angle Pairs, Lines and Planes

2. 3.1 Parallel and Perpendicular Lines Parallel lines are coplanar lines that do not intersect. Intersecting lines are coplanar and have exactly one point in common If intersecting lines meet at right angles, they are perpendicular, otherwise they are oblique. > > Perpendicular lines Obilque lines Parallel lines Algebraic Property Two non-vertical lines are parallel if and only if they have the same slope.

3. 3.1 Transitive Property of Parallel Lines Let l1 , l2, l3 be three coplanar lines. Prove that if l1 is parallel to l2 and l2 is parallel to l3 then l1 is parallel to l3. Solution: one way to write a proof is to superimpose A coordinate plane over the three lines. Do this in a way so that none of the lines is vertical as shown. Given: l1║l2 and l2║l3 Prove: l1║l3 Let m1, m2 and m3 be the slopes of l1 , l2, l3 Because l1 and l2 have the same slope, they are parallel. l2 l1 l3 > > > Theorem 3.1 Transitivity of Parallel Lines If two lines are parallel to the same line, then they are parallel to each other

4. 3.1 A Property of Perpendicular Lines Let l1 , l2, l3 be coplanar. Prove that if l1 is perpendicular to l2 and l2 is perpendicular to l3 then l1 is parallel to l3. Solution: one way is to superimpose a coordinate plane over the three lines. Do this in a way so that none of the lines is vertical or horizontal as shown. Given: l1 l2 and l2 l3 Prove: l1l3 Let m1, m2 and m3 be the slopes of l1 , l2, l3 Because l1 and l2 have the same slope, they are parallel. l2 l1 l3 Theorem 3.2 Property of Perpendicular Lines If two coplanar lines are perpendicular to the same line, then they are parallel to each other.

5. 3.1 Algebraic Property 2 non-vertical lines are parallel if and only if they have the same slope Theorem 3.1 Transitivity of Parallel Lines If 2 lines are parallel to the same line, then they are parallel to each other. Algebraic Property 2 non-vertical lines are parallel if and only if they have the same slope Theorem 3.2 Property of Perpendicular Lines If 2 coplanar lines are perpendicular to the same line, then they are parallel to each other Geometric Definition Skew lines are lines that do not lie in the same plane Geometric Definition Parallel planes are planes that do not intersect

6. 3.1 Proving that 2 lines are Parallel t Postulate 16 Corresponding Angles ConverseIf two lines are cut by a transversal so that corresponding angles are congruent, then the lines are parallel 5 1 l 4 3 6 2 m 8 7 Alternate Interior Angles Converse if two lines are cut by a transversal so that alternate interior angles are congruent, then the lines are parallel Consecutive Interior Angles Converse if two lines are cut by a transversal so that consecutive interior angles are supplementary , then the lines are parallel Alternate Exterior Angles Converse if two lines are cut by a transversal so that alternate exterior angles are congruent, then the lines are parallel

7. 3.2 Solving Systems of Linear Equations In Algebra, 3 possibilities exist for a system of 2 linear equations: One solution where the lines intersect No solution where lines are parallel Infinite solutions where the lines are the same line (called “coincident lines”) • Postulate 12. If 2 distinct lines intersect, then their intersection is exactly one point.

8. 3.2 Solving Systems of Linear Equations Example 1: Solve the system, x – 2 y = – 7 3 x + 4 y = 9 Solution: Begin by solving one of the equations for one of the variables. Then substitute into the other equation. • (– 1, 3 ) 3x + 4y = 9 x – 2 y = 7 By substituting y = 3 into the first equation, you obtain x = – 1. Check this solution by substituting x = – 1 and y = 3 into both of the original equations. You can also check the solution graphically, as shown to the right.

9. 3.2 Finding a Parallel Line Example 2: The line l1 is given by y = – 3x + 2. The line l2 is parallel to line l1 and passes through the point (2, 1). Find an equation for l2 Solution: Line l1 has a slope of – 3. Because parallel lines have the same slope, you know that l2 also has a slope of – 3 . You can find the y-intercept of l2 as follows: l2 l1 6 Now that you know the slope and y-intercept of l2 you can write its equation. 2 (2, 1) • y = – 3 x + 2 Check that (2, 1) is on the line. The graph of l2 is shown to the right.

10. 3.2 Finding a Perpendicular Line Example 3: The line l1 is given by y = – 2x + 1. The line l2 is perpendicular to line l1 and passes through the point (4, 0). Find an equation for l2 Solution: Line l1 has a slope of – 2. Because perpendicular lines have slopes that are negative reciprocals, you know that l2 also has a slope of ½ . You can find the y-intercept of l2 using the slope-intercept form as follows: l1 y = – 2 x + 1 3 (4, 0) 1 Now that you know the slope and y-intercept of l2 you can write its equation. • l2 The graph of l2 is shown to the right.

11. 3.2 Relationships among lines B C A D F G H E AD and FG are parallel AD and AE are perpendicular AD and HG are skew

12. 3.2 Theorem: Through a given point on a line, there exists exactly one perpendicular to the given line, L Perpendicular line L ● Theorem: Through a given point NOT on a line, there exists exactly one perpendicular to the given line, L Perpendicular line ● L

13. 3.5 Angles Formed by a Transversal A transversalis a line that intersects two or more coplanar lines at different points. t Angles Formed by a Transversal In the figure at the right, the transversal t intersects the lines l and m Two angles are corresponding angles if they occupy corresponding positions, such as, < 1 and < 5 Two angles are alternate interior angles if they lie between l and m on opposite sides of t, such as < 2 and < 8 Two angles are alternate exterior angles if they lie outside l and m on opposite sides of t, such as < 1 and < 7 Two angles are consecutive interior angles if they lie between l and m on same side of t, such as < 2 and < 5 4 1 3 2 l 8 5 m 7 6

14. 3.5 Angles Formed by a Transversal t Postulate 15 Corresponding Angles Postulate. If two parallel lines are cut by a transversal, then the pairs of corresponding angles are congruent. 5 1 l 4 3 6 2 m 8 7 Alternate Interior Angles Theorem if two parallel lines are cut by a transversal, then the pairs of alternate interior angles are congruent Consecutive Interior Angles Theorem if two parallel lines are cut by a transversal, then the pairs of consecutive interior angles are supplementary Alternate Exterior Angles Theorem if two parallel lines are cut by a transversal, then the pairs of alternate exterior angles are congruent Perpendicular Transversal Theorem if a transversal is perpendicular to one of two parallel lines, then it is perpendicular to the second

15. 3.6 Proof of “Alternate Interior Angles Converse” 3 L 1 1 2 L 2 t

16. 3.7 VECTORS A Vector is a quantity that has both a direction and magnitude (length). Vectors are represented by directed line segments from an initial point to a terminal point. 2 Vectors are parallel if the have the same or opposite directions NOTE: a ray and a vector both have an INITIAL point but a vector also has a TERMINAL point whereas a ray does not. 2 Vectors are equal if they have the same length and direction, however they don’t have to have the same initial and terminal points. • • • terminal ray vector • • • initial • initial

17. 3.7 VECTORS (x2 , y2) Q • (x1 , y1) • • P ( x2 ─ x1 , y2 ─ y1 ) (0, 0) • Let P (x1 , y1) and Q (x2 , y2) be points in a coordinate plane. The Vector PQ can be represented by the directed line segment whose initial point is ( 0, 0 ) and whose terminal point is ( x2 ─ x1 , y2 ─ y1 ). This ordered pair representation is denoted by v = ( x2 ─ x1 , y2 ─ y1 ).

18. 3.7 Writing a vector as an ordered pair (3, 4) Let P (3, 4) and Q (4, 2) Use Distance Formula: PQ = ( 4─ 3 ) 2 + ( 2─ 4) 2 • P (4, 2) • Q (0, 0) • PQ = 5 5 • ( 1, ─ 2) Let P (3, 4) and Q (4, 2) Find the ordered pair representation of PQ where the initial point is ( 0, 0 ) and the terminal point is ( 1, ─ 2) The Vector PQ = ( x2 ─ x1 , y2 ─ y1 ) = ( 4─3 , 2─ 4) = ( 1 , ─ 2 ) denoted by v = ( x2 ─ x1 , y2 ─ y1 ).

19. 3.7 ADDING VECTORS • u Two vectors can be added to form a new vector using the parallelogram rule: To geometrically add u and v, move the initial point of u to the terminal point of v or vice versa. Note that the sum of u + v is the DIAGONAL of a parallelogram. (1, 5) • v v + u v • (4, 2) (0, 0) u • Algebraically: the sum of v = ( a1 , b1 ) and u = ( a2 , b2 ) is v + u = (a1 + a2 , b1 + b2 ) If v + u = ( 0 , 0 ), the v and u have the same length but opposite directions Example above: the sum of v = ( 1, 5 ) and u = ( 4, 2 ) is v + u = ( 1+ 4 , 5+ 2 ) = ( 5 , 7 ) If v + u = ( 0 , 0 ), the v and u have the same length but opposite directions

20. 3.7 MULTIPLYING VECTORS Two vectors can be multiplied using the operation of dot product. The result is a real number, NOT a vector. Let v = ( a1 , b1 ) and u = ( a2 , b2 ). The DOT PRODUCT of these 2 vectors is v ● u = a1 b1 + a2 b2 Two non-zero vectors are perpendicular if and only if their dot product is zero. • v = ( 5, 10 ) and u = ( 4, ─ 2 ) v ● u = (5) (4) + (10) (─ 2) = 20 + ─ 20 = 0 v and u are ( 5, 10 ) • • (4, ─ 2 )

21. To find: Ordered Pair Representation given an initial point (1,5 ) and terminal point (4,2) SUBTRACT the initial coordinates from the terminal coordinates (4 – 1 , 2 – 5) (3 , – 3) To find: SUM of 2 VECTORS: v = (1,5) and u = (4,2) ADD the x coordinates and y coordinates to get SUM coordinates v + u = (a1 + a2 , b1 + b2 ) v + u = ( 1+ 4 , 5+ 2 ) = (5 ,7) To find: DOT PRODUCT of 2 VECTORS: v = (1,5) and u = (4,2) MULTIPLY the x coordinates and, then multiply the y coordinates and then ADD both to get the Product v • u = (a1 • a2 , b1 • b2 ) v • u = ( 1• 4+ 5• 2 ) = 14

22. Distance: The term distance in geometry is always interpreted as the shortest path between two points A B ● ● Distance: The shortest distance between two points is a straight line.