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# 3.2 theorems about perpendicular lines

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### Transcript

• 1. Theorems About Perpendicular Lines
• 2. Comparing Types of Proofs
• Two-Column Proof
• Paragraph Proof
• Flow Proof
• Uses arrows to show the flow of the logical argument
• 3. Right Angle Congruence Theorem 1. Two Column Proof: All right angles are congruent. Given : 1 and 2 are right angles Prove : 1 ≅ 2 Statement Reason 1 and 2 are right angles Given m 1 = 90 o , m 2 = 90 o Definition of a right angle m 1 = m 2 Transitive property of equality 1 ≅ 2 Definition of congruent angles
• 4.
• Because angles 1 and 2 are right angles, their measures are equal to 90 o , by the definition of right angles. Hence by the transitive property of equality, the measure of angle 1 is equal to the measure of angle 2. By the definition of congruent angles, angle 1 is congruent to angle 2.
2. Paragraph Proof
• 5. 3. Flow Proof
• 6. Theorem 3.1
• If two lines intersect to form a linear pair of congruent angles, then the lines are perpendicular.
• 7. Theorem 3.2
• If two sides of two adjacent acute angles are perpendicular, then the angles are complementary.
• 8. Theorem 3.3
• If two lines are perpendicular, then they intersect to form four right angles.
• 9. Proof of Theorem 3.2 Prove : 1 + 2 are complementary Statement Reason AB BC Given ABC is a right angle Definition of perpendicular lines m ABC = 90 o Definition of a right angle m 1 + m 2 = m ABC Angle addition postulate m 1 + m 2 = 90 o Substitution property of equality 1 + 2 are complementary Definition of complementary angles
• 10. Practice and Homework
• Workbook: Exercise 3.2
• Textbook: Exercise 3.2 p138: 1-27, odd