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Geometry 3.1: Parallel Lines & Transversals

• Parallel lines are coplanar lines that do not intersect. AT ALL. EVER.• Skew lines are lines that do not intersect and are not coplanar.• Parallel planes are planes that do not intersect.

For example:

J

L

K

M

JK || LM(Arrows are used to

indicate that lines are parallel.)

l

m

A

B

Lines l and m are skew.Planes A and B are parallel.

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Transversal Angle Pair Relationships

Four interior angles lie in the region between lines q and r.

Angles 3, 4, 5, and 6.

Four exterior angles lie in the two regions that are not between lines q and r.

Angles 1, 2, 7, and 8.

Consecutive interior angles are interior angles that lie on the same side of transversal t.

Angles 4 & 5; angles 3 & 6.

Alternate interior angles are nonadjacent interior angles that lie on opposite sides of transversal t.

Angles 3 & 5; angles 4 & 6.

Alternate exterior angles are nonadjacent exterior angles that lie on opposite sides of transversal t.

Angles 1 & 7; angles 2 & 8.

Corresponding angles lie on the same side of transversal t and on the same side of lines q and r.

Angles 1 & 5; angles 2 & 6; angles 3 & 7; angles 4 & 8.

t

r

exterior

exterior

interior

12

34

5 678

q

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Geometry Section 3-2: Angles and Parallel Lines

Objectives:1) Use theorems to determine the relationships between specific pairs of angles.2) Use algebra to find angle measures.

Corresponding Angles Postulate 3.1- if 2 parallel lines are cut by a transversal, then each pair of corresponding angles are congruent. 1≅ 5; 2≅ 6; 3≅ 7; 4≅ 8

Alternate Interior Angle Theorem 3.1- If 2 parallel lines are cut by a transversal, then each pair of alternate interior angles are congruent. 3≅ 6; 4≅ 5

1 23 4

5 6

7 8

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Consecutive Interior Angles Theorem 3.2- If 2 parallel lines are cut by a transversal, then each pair of consecutive interior angles is supplementary.

3 and 5 are supplementary.4 and 6 are supplementary.

Alternate Exterior Angles Theorem 3.3- If 2 parallel lines are cut by a transversal, than each pair of alternate exterior angles are congruent.

5≅ 7 and 6≅ 8

1 23 4

5 6

7 8

5

a

b

t

Perpendicular Transversal Theorem 3.4 - In a plane, if a line is perpendicular to one of two parallel lines, then it is perpendicular to the other.

If line a || line b and line a line t, then line b line t.

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Auxiliary Lines

An extra line or segment drawn in a figure to help complete a proof.

xo

105o

125o

To find x, we can draw in a parallel auxiliary line to help out.

auxiliary line

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Geometry Section 3-3 Slopes of Lines

Objectives: (1) Find the slope of a line.(2) Use slope to identify parallel and perpendicular lines.

In a coordinate plane, the slope of a line is the ratio of change along the y-axis to the change along the x-axis between any two points on the line.

The slope m of a line containing two points with coordinates (x1,y1) and (x2,y2) is given by the formula:

y2-y1 x2-x1

m= , where x1≠x2

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positiveslope

undefinedslope

negativeslope

zeroslope

Classifying Slopes

Slopes can be interpreted as a rate of change, describing how a quantity y changes in relationship to quantity x.

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Postulates for Parallel & Perpendicular Lines

3.2 Slopes of Parallel Lines:Two non vertical lines have the same slope if and only if (iif) they are parallel. All vertical lines are parallel.

3.3 Slopes of Perpendicular Lines: Two non vertical lines are perpendicular if and only if (iif) the product of their slopes is -1. Vertical and horizontal lines are perpendicular.

Note: If and only if both the conditional and its converse are true it can be written in if and only if form or with an iff.

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Geometry Section 3.4 Equations of Lines

Objectives(1) Write an equation of a line given information about the graph.(2) Solve problems by writing equations.

The slope-intercept form of a linear equation is y = mx + b, where m is the slope of the line and b is the y-intercept.

The point-slope form of a linear equation is y - y1 = m(x - x1), where (x1,y1) is any point on the line and m is the slope of the line.

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Horizontal and Vertical Line Equations

The equation of a horizontal line is y = b, where b is the y-intercept of the line.

The equation of a vertical line is x = a, where a is the x-intercept of the line.

x = -3

y = -4

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Geometry Section 3.5Proving Lines Parallel

Objectives1) Recognize angle pairs that occur with parallel lines.2) Prove that two lines are parallel using angle relationships.

Postulate 3.4: If two lines in a plane are cut by a transversal so that corresponding angles are congruent, then the lines are parallel.

Parallel Postulate 3.5: If given a line and a point not on the line, then there exists exactly one line through the point that is parallel to the given line.

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Theorem 3.5 (Alternate Exterior Angles Converse): If two lines in a plane are cut by a transversal so that a pair of alternate exterior angles are congruent, then the two lines are parallel.

Theorem 3.6 (Consecutive Interior Angles Converse): If two lines in a plane are cut by a transversal so that a pair of consecutive interior angles is supplementary, then the two lines are parallel.

Theorem 3.7 (Alternate Interior Angles Converse): If two lines in a plane are cut by a transversal so that a pair of alternate interior angles is congruent, then the two lines are parallel.

Theorem 3.8 (Perpendicular Transversal Converse): In a plane, if two lines are perpendicular to the same line, then they are parallel.

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Geometry 3.6: Perpendiculars & Distance

Objectives(1) Find the distance between a point and a line.(2) Find the distance between parallel lines.

Distance Between a Point and a Line:The distance between a line and point not on the line is the length of the segment perpendicular to the line from the point.

distance from the point to line

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(3,8)

(0,2)

Distance Formula:

Where (x1,y1) and (x2,y2) are points on a line.

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Postulate 3.6 (Perpendicular Postulate): If given a line and a point not on the line, then there exists exactly one line through the point that is perpendicular to the given line.

P

m

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Equidistant: The distance between two lines measured along a perpendicular line is always the same.

Distance between Parallel Lines: The distance between two parallel lines is the perpendicular distance between one of the lines and any point on the other line.

Theorem 3.9 (Two Lines Equidistant from a Third): In a plane, if two lines are each equidistant from a third line, then the two lines are parallel to each other.

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HomeworkChapter 3 Cumulative

Review WKS