Geometry: Chapter 3 Ch. 3. 4: Prove Lines are Parallel Ch. 3.5 Using Properties of Parallel Lines.

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Geometry: Chapter 3

Ch. 3. 4: Prove Lines are ParallelCh. 3.5 Using Properties of Parallel Lines

Postulate 16: Corresponding Angles Converse

If two lines are cut

by a transversal so

the corresponding

angles are congruent,

then the lines are parallel. Image taken from: Geometry. McDougal Littell: Boston, 2007. P. 161.

Ex. 1. Find the value of y that makes a || b.

Image taken from: Geometry. McDougal Littell: Boston, 2007. P. 162.

Ex. 1 (cont.)

Solution: Lines a and b are parallel if the marked alternate exterior angles are congruent.

(5y +6)o =121o 5y=121-65y=115

y = 23

Theorem 3.8: Alternate Interior Angles Converse

If two lines are cut by a transversal so the alternate interior angles are congruent, then the lines are parallel.

Theorem 3.9: Consecutive Interior Angles Converse

If two lines are cut by a transversal so the consecutive interior angles are supplementary,then the lines are parallel.

Theorem 3.10: Alternate Exterior Angles Converse

If two lines are cut by a transversal so the alternate exterior angles are congruent, then the lines are parallel.

Example 2: A woman was stenciling this design on her kitchen walls. How can she tell if the top and bottom are parallel?

She can measure alternate interior angles or corresponding angles and see if they are congruent.

Ex. 3: Prove that if 1 and 4 are supplementary, then a||b.

Ex. 4: In the figure, a || b and 1 is congruent to 3. Prove c || d. Use a paragraph proof.

Theorem 3.11: Transitive Property of Parallel Lines.

If two lines are parallel to the same line, then they are parallel to each other.

Theorem 3.12: Lines Perpendicular to a Transversal Theorem

In a plane, if two lines are perpendicular to the same line, then they are parallel to one another.

If m ┴ p and n┴ p, then m || n.

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