# Chapter 9 Parallel Lines Section 1 Proving Lines

- Slides: 58

Chapter 9 Parallel Lines Section 1: Proving Lines Parallel C. N. Colon St. Barnabas H. S. Geometry - HP

p Parallel Lines n Parallel lines are lines that are coplanar and do not intersect.

Some examples of parallel lines

p Parallel Planes n Planes that do not intersect Parallel capacitors

Parallel Postulate p If there is a line and a point not on the line, then there is exactly one line through the point parallel to the given line. p P m There is exactly one line through P parallel to m.

p Transversal n A line that intersects two or more coplanar lines at different points

p Corresponding Angles Postulate If two parallel lines are cut by a transversal, then the pairs of corresponding angles are congruent. 1 2 1 � 2

p Alternate Interior Angles Theorem If two parallel lines are cut by a transversal, then the pairs of alternate interior angles are congruent. 3 4 3� 4

p Consecutive Interior Angles Theorem If two parallel lines are cut by a transversal, then the pairs of consecutive interior angles are supplementary. 5 6 m 5 + m 6 = 180°

p Alternate Exterior Angles Theorem If two parallel lines are cut by a transversal, then the pairs of alternate exterior angles are congruent. 7 8 7 � 8

p Perpendicular Transversal Theorem n n If a transversal is perpendicular to one of two parallel lines, then it is perpendicular to the other. If two coplanar lines are each perpendicular to the same line then they are parallel j h k If j is perpendicular to line k and to line h, then lines h and k are parallel

p Example 1 n n Given: p || q with a transversal Prove: m 1 = m 8 Statements 3 1 5 6 7 8 4 2 Reasons 1. 2. 3. 4. p q

Given that m 5 = 65°, find each measure. Tell which postulate or theorem you used to find each q one. p a. c. b. d. 6 7 9 5 8

How many other angles have a measure of 100°? AB || CD AC || BD B A 100° D C

Use properties of parallel lines to find the value of x. (x – 8)° 72°

p Find the value of x. x° 70° (x – 20)°

Proving Lines are Parallel Corresponding Angle Converse Postulate p If two lines are cut by a transversal so that corresponding angles are congruent, then the lines are parallel j j || k k

Proving Lines are Parallel Alternate Interior Angles Converse p If two lines are cut by a transversal so that alternate interior angles are congruent then the lines are parallel. j 3 1 k If 1 3, then j || k

Proving Lines are Parallel Consecutive Interior Angles Converse n If two lines are cut by a transversal so that consecutive interior angles are supplementary, then the two lines are parallel j 2 1 k If m 1 + m 2 = 180°, then j || k

Proving Lines are Parallel Alternate Exterior Angles Converse n If two lines are cut by a transversal so that alternate exterior angles are congruent, then the lines are parallel. 4 j k 5 If 1 3, then j || k.

Proving Lines are Parallel n n Given: m p, m Prove: p || q m p 1 q 2 Statements Reasons 1. 2. 3. 4. q

Proving Lines are Parallel p Example 2 n n A 4 B 5 Given: 5 6, 6 4 Prove: AD || BC 6 D C

Find the value of x that makes m || n. m n (2 x + 1)° (3 x – 5)°

n n Is AB || DC? Is BC || AD? 155° D 65° 40° 65° A B 115° C

When the lines r and s are cut by a transversal, 1 and 2 are same side interior angles. If m 1 is three times m 2, can r be parallel to line s? Explain

Remember: p p p The sum of the interior degrees of a triangle is ___180°___. The sum of the degrees of a pair of complementary angles is ___90°___. The sum of the degrees of a pair of supplementary angles is ___180°___. The sum of the degrees of consecutive interior angles if transversal crosses parallel lines is ___180°___. Parallel lines have slopes that are congruent.

Section 9 -2 Properties of Parallel Lines

Using Properties of Parallel Lines p Lines Parallel to a Third Line Theorem n If two lines are parallel to the same line, then they are parallel to each other. p q r If p || q and q || r, then p ||r

Using Properties of Parallel Lines p Lines Perpendicular to a Third Line Theorem n In a plane, if two lines are perpendicular to the same line, then they are parallel to each other. m n If m p p and n p, then m ||n

Using Properties of Parallel Lines p 1 Example 1 n n 2 Given: r || s and s || t Prove: r || t 4 3 Statements Reasons 1. 2. 3. 4. 5. 6. r s t

Using Properties of Parallel Lines p Example 2 n The flag of the United States has 13 alternating red and white stripes. Each stripe is parallel to the stripe immediately below it. Explain why the top stripe is parallel to the bottom stripe. S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 S 11 S 12 S 13 Describe your thinking as you prove that S 1 and s 13 are parallel

Using Properties of Parallel Lines Example 3 You are building a CD rack. You cut the sides, bottom, and top so that each corner is composed of two 45 o angles. Prove that the top and bottom front edges of the CD rack are parallel. Given: Prove: Angle Addition Postulate Given Substitution Property Definition of a right angle Definition of perpendicular lines Angle Addition Postulate Given Substitution Property Definition of perpendicular lines In a plane, 2 lines ⊥ to the same line are ║

Geometry 1 Unit 3 3. 6 Parallel Lines in the Coordinate Plane

Parallel Lines in the Coordinate Plane The slope of a line is usually represented by the variable m. Slope is the change in the rise, or vertical change, over the change in the run, or horizontal change.

Parallel Lines in the Coordinate Plane p Example 1 n Cog railway A cog railway goes up the side of a Mount Washington, the tallest mountain in New England. At the steepest section, the train goes up about 4 feet for each 10 feet it goes forward. What is the slope of this section. rise =____ run =____ slope = -------- = -------

Parallel Lines in the Coordinate Plane p Example 2 n The cog railway covers about 3. 1 miles and gains about 3600 feet of altitude. What is the average slope of the track?

Parallel Lines in the Coordinate Plane p Example 3 n x 1= x 2= Find the slope of a line that passes through the points (0, 6) and (5, 2). y 1 = y 2 = slope = ----------

Parallel Lines in the Coordinate Plane p Slopes of Parallel Lines Postulate n In a coordinate plane, two non-vertical lines are parallel if and only if they have the same slope. Any two vertical lines are parallel.

Parallel Lines in the Coordinate Plane Example 4 Find the slope of each line.

Parallel Lines in the Coordinate Plane Example 5 Find the slope of each line. Which lines are parallel?

Parallel Lines in the Coordinate Plane p In algebra, you learned that you can use the slope m of a non-vertical line to write the equation of the line in slope intercept form. y-intercept y = mx + b slope

Parallel Lines in the Coordinate Plane p Example 6 n y = 2 x + 5 y = -½x – 3 What is the slope? p What is the y-intercept? p Do you have enough information to graph the line? p

Parallel Lines in the Coordinate Plane Example 7 Write the equation of a line through the point (2, 3) with a slope of 5. Step 1: x= y= m= Step 2: Substitute the values above into the equation y = mx + b. SOLVE FOR b. ______ = (_______) (_____) + b y m x Step 3 Rewrite the equation of the line in slope-intercept form, using m and b from your solution to the equation above y = _______ x + _____ m b

Parallel Lines in the Coordinate Plane p Example 8 n n n Line k 1 has the equation y = 2/5 x + 3. Line k 2 is parallel to k 1 and passes through the point (-5, 0). Write the equation of k 2.

Parallel Lines in the Coordinate Plane p Example 9 n Write an equation parallel to the line n What do you have to keep the same as the original equation? What did you change? n

Parallel Lines in the Coordinate Plane p Example 10 n n A zip line is a taut rope or a cable that you can ride down on a pulley. The zip line below goes from a 9 foot tall tower to a 6 foot tower 20 feet away. What is the slope of the zip line?

Geometry 1 Unit 3 3. 7 Perpendicular Lines in the Coordinate Plane

Perpendicular Lines in the Coordinate Plane Activity: Investigating Slope of Parallel Lines You will need: an index card, a pencil and the graph below. Place the index card at any angle – except straight up and down – on the coordinate plane below, with a corner of the card placed on an intersection. Use the edge of the card like a ruler, draw to lines, that will intersect at the corner of the card that lines up with the intersection on the coordinate plane. Name the lines ‘o’ and ‘p’. Move the index card and select, then label, two points on line. These should be points where the line goes directly through an intersection on the coordinate plane. Using the equation for slope, find the slope of each line.

Perpendicular Lines in the Coordinate Plane Example 1 Label the point of intersection And the x-intercept of each line. Find the slope of each line. Multiply the slopes. Question: What do you notice? Look at the activity from the start of class. Multiply the slopes of those lines. Question: What do you notice? What is true about the product of the slopes of perpendicular lines?

Perpendicular Lines in the Coordinate Plane Example 2 Decide whether and are perpendicular. A D C B What is the product of the slopes of perpendicular lines? _____________ Are these lines perpendicular? ______

Perpendicular Lines in the Coordinate Plane Example 3 A Decide whether and B are perpendicular. C D What is the product of the slopes of perpendicular lines? _____________ Are these lines perpendicular? ______

Perpendicular Lines in the Coordinate Plane Example 4 Decide whether these lines are perpendicular. line h: line j: What is the product of the slopes of perpendicular lines? _____________ Are these lines perpendicular? ______

Perpendicular Lines in the Coordinate Plane Example 5 Decide whether these lines are perpendicular. line r: line s: What is the product of the slopes of perpendicular lines? _____________ Are these lines perpendicular? ______

Perpendicular Lines in the Coordinate Plane Slope of a line 7 4 -1 Slope of the perpendicular line Product of the slopes

Perpendicular Lines in the Coordinate Plane Example 6 Line l 1 has equation y = -2 x +1. Find an equation for the line, l 2 that passes through point (4, 0) and is perpendicular to l 1. What is the slope of l 1? _______ What form is l 1 written in? ________________ What does the slope of l 2 need to be if they are perpendicular? _____ With the point known (4, 0) , (it is in the original question), and the slope known for l 2 , Can you find the y-intercept, b, of the perpendicular line? x = ________ y = ________ What is the equation of the perpendicular line? m = ________ b = _______

Perpendicular Lines in the Coordinate Plane Example 7 Line g has equation y = 3 x - 2. Find an equation for the line h that passes through point (3, 4) and is perpendicular to g. What is the slope of g? _______ What form is g written in? ________________ What does the slope of h need to be if they are perpendicular? _____ With the point known (3, 4), (it is in the original question), and the slope known for h , Can you find the y-intercept, b, of the perpendicular line h? x = ________ y = ________ What is the equation of line h? m = ________ b = _______

Perpendicular Lines in the Coordinate Plane Example 8 What is the equation of a line a, which passes through point (-2, 0) that is perpendicular to line z, What is the slope of z? _______ What form is z written in? ________________ What does the slope of a need to be if they are perpendicular? _____ With the point known (-2, 0) , (it is in the original question), and the slope known for z , Can you find the y-intercept, b, of the perpendicular line? x = ________ y = ________ m = ________ b = ________ What is the equation of the perpendicular line? ___________

Perpendicular Lines in the Coordinate Plane Example 9 Line g has equation . Find an equation for the line s that passes through point (3, 1) and is perpendicular to g. What is the slope of g? _______ What form is g written in? ________________ What does the slope of s need to be if they are perpendicular? With the point known (3, 1) , what is the equation of the perpendicular line s? x = ________ y = ________ m = ________ b = ________

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