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

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slide1

Angles Formed by Parallel Lines and Transversals

3-2

Warm Up

Lesson Presentation

Lesson Quiz

Holt Geometry

slide2

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

slide3

Objective

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

slide5

Example 1: Using the Corresponding Angles Postulate

Find each angle measure.

A. mECF

x = 70

Corr. s Post.

mECF = 70°

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°

slide6

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°

slide7

Helpful Hint

If a transversal is perpendicular to two parallel lines, all eight angles are congruent.

slide8

Remember that postulates are statements that are accepted without proof.

Since the Corresponding Angles Postulate is given as a postulate, it can be used to prove the next three theorems.

slide9

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°

slide10

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.

slide11

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

Subtract the first equation from the second equation.

Substitute 5 for y in 5x + 5y = 60. Simplify and solve for x.

5x + 5(5) = 60

x = 7, y = 5

slide12

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°.

slide13

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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