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Angles Formed by Parallel Lines and Transversals

Learn about angles formed by parallel lines and transversals through a warm-up presentation and quiz. Understand theorems related to corresponding, alternate exterior and interior angles, and consecutive interior angles. Practice finding angle measures using postulates and theorems.

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Angles Formed by Parallel Lines and Transversals

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  1. Angles Formed by Parallel Lines and Transversals 3-2 Warm Up Lesson Presentation Lesson Quiz Holt Geometry

  2. Warm Up Identify each angle pair. 1.1 and 3 2. 3 and 6 3. 4 and 5 4. 6 and 7 corr. s alt. int. s alt. ext. s same-side int s

  3. Objective Prove and use theorems about the angles formed by parallel lines and a transversal.

  4. What is the difference between the two figures? These lines appear to be parallel

  5. Figure A Figure B What do you notice about the corresponding angles in both figures? In figure B, one angle looks like it is obtuse and the other is acute-y In figure A, the angles look like they are the same measure

  6. When a transversal slashes two PARALLEL lines, the following theorems are true. Make sure you know these theorems.

  7. Reminder: we are strictly talking parallel lines here… 1 3 What do you think is the relationship between these two angles? What kind of angles are 1 and 3? Corresponding They’re congruent!

  8. Reminder: we are strictly talking parallel lines here… So… the Corresponding Angles Postulate is that if two parallel lines are cut by a transversal, then ALL of the pairs of corresponding angles are congruent...Wow!

  9. Reminder: we are strictly talking parallel lines here… What is the relationship between alternate exterior angles? They’re congruent

  10. Reminder: we are strictly talking parallel lines here… What is the relationship between alternate interior angles? They’re congruent

  11. Reminder: we are strictly talking parallel lines here… What is the relationship between consecutive interior angles? They’re supplementary

  12. Example 1: Using the Corresponding Angles Postulate Find each angle measure. A. mECF mECF = 70° Corr. s Post. B. mDCE 5x = 4x + 22 Corr. s Post. x = 22 Subtract 4x from both sides. mDCE = 5x = 5(22) Substitute 22 for x. = 110°

  13. Check It Out! Example 1 Find mQRS. x = 118 Corr. s Post. mQRS + x = 180° Def. of Linear Pair Subtract x from both sides. mQRS = 180° – x = 180° – 118° Substitute 118° for x. = 62°

  14. Let’s think… If a transversal is perpendicular to two parallel lines, all eight angles are congruent. (all 90 degrees)

  15. Example 2: Finding Angle Measures Find each angle measure. A. mEDG mEDG = 75° Alt. Ext. s Thm. B. mBDG x – 30° = 75° Alt. Ext. s Thm. x = 105 Add 30 to both sides. mBDG = 105°

  16. Check It Out! Example 2 Find mABD. 2x + 10° = 3x – 15° Alt. Int. s Thm. Subtract 2x and add 15 to both sides. x = 25 mABD = 2(25) + 10 = 60° Substitute 25 for x.

  17. Example 3: Music Application Find x and y in the diagram. By the Alternate Interior Angles Theorem, (5x + 4y)° = 55°. By the Corresponding Angles Postulate, (5x + 5y)° = 60°. -(5x + 5y = 60) + (5x + 4y = 55) y = 5 Add the first equation to the second equation. Substitute 5 for y in 5x + 5y = 60. Simplify and solve for x. 5x + 5(5) = 60 x = 7, y = 5

  18. Check It Out! Example 3 Find the measures of the acute angles in the diagram. By the Alternate Exterior Angles Theorem, (25x + 5y)° = 125°. By the Corresponding Angles Postulate, (25x + 4y)° = 120°. An acute angle will be 180° – 125°, or 55°. The other acute angle will be 180° – 120°, or 60°.

  19. Lesson Quiz State the theorem or postulate that is related to the measures of the angles in each pair. Then find the unknown angle measures. 1. m1 = 120°, m2 = (60x)° 2. m2 = (75x – 30)°, m3 = (30x + 60)° Alt. Ext. s Thm.; m2 = 120° Corr. s Post.; m2 = 120°, m3 = 120° 3. m3 = (50x + 20)°, m4= (100x – 80)° 4. m3 = (45x + 30)°, m5 = (25x + 10)° Alt. Int. s Thm.; m3 = 120°, m4 =120° Same-Side Int. s Thm.; m3 = 120°, m5 =60°

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