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Geometry Postulates and Proofs

Learn about postulates in geometry and how to write algebraic and geometric proofs. Practice solving equations and proving geometric concepts.

wendybaker
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Geometry Postulates and Proofs

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  1. In the figure shown, A, C, and lie in plane R, and B is on . Which option states the postulate that can be used to show that A, B, and C are collinear? A. A line contains at least two points. B. A line contains only two points. C. A line contains at least three points. D. A line contains only three points. 5-Minute Check 1

  2. In the figure shown, A, C, and lie in plane R, and B is on . Which option states the postulate that can be used to show that lies in plane R? A. Through two points, there is exactly one line in a plane. B. Any plane contains an infinite number of lines. C. Through any two points on the same line, there is exactly one plane. D. If two points lie in a plane, then the entire line containing those points lies in that plane. 5-Minute Check 2

  3. In the figure shown, A, C, and lie in plane R, and B is on . Which option states the postulate that can be used to show that A, H, and D are coplanar? A. Through any two points on the same line, there is exactly one plane. B. Through any three points not on the same line, there is exactly one plane. C. If two points lie in a plane, then the entire line containing those points lies in that plane. D. If two lines intersect, then their intersection lies in exactly one plane. 5-Minute Check 3

  4. In the figure shown, A, C, and lie in plane R, and B is on . Which option states the postulate that can be used to show that E and F are collinear? A. Through any two points, there is exactly one line. B. A line contains only two points. C. If two points lie in a plane, then the entire line containing those points lies in that plane. D. Through any two points, there are many lines. 5-Minute Check 4

  5. In the figure shown, A, C, and lie in plane R, and B is on . Which option states the postulate that can be used to show that intersects at point B? A. The intersection point of two lines lies on a third line, not in the same plane. B. If two lines intersect, then their intersection point lies in the same plane. C. The intersection of two lines does not lie in the same plane. D. If two lines intersect, then their intersection is exactly one point. 5-Minute Check 5

  6. A.–7 B. C. D.34 Which of the following numbers is an example of an irrational number? 5-Minute Check 6

  7. Homework/Quiz questions?

  8. You used postulates about points, lines, and planes to write paragraph proofs. • Use algebra to write two-column proofs. • Use properties of equality to write geometric proofs. Then/Now

  9. algebraic proof • two-column proof • formal proof Vocabulary

  10. Concept

  11. Justify Each Step When Solving an Equation Solve 2(5 – 3a) – 4(a + 7) = 92. Algebraic Steps Properties 2(5 – 3a) – 4(a + 7) = 92 Given 10 – 6a – 4a – 28 = 92 Distributive Property –18 – 10a = 92 Combine Like Terms –18 – 10a + 18 = 92 + 18 Addition Property Example 1

  12. Justify Each Step When Solving an Equation –10a = 110 Simplify Division Property a = –11 Simplify Answer:a = –11 Example 1

  13. Solve –3(a + 3) + 5(3 – a) = –50. A.a = 12 B.a = –37 C.a = –7 D.a = 7 Example 1

  14. Write an Algebraic Proof Begin by stating what is given and what you are to prove. Example 2

  15. Proof: d = 20t + 5 1. Given 1. Statements Reasons 2. d – 5 = 20t 2. Subtraction Prop. of Equality = t 3. 3. Division Property of Equality 4. 4. Symmetric Prop. of Equality Write an Algebraic Proof Example 2

  16. If the formula for the area of a trapezoid is , then the height h of the trapezoid is given by . Which of the following statements would complete the proof of this conjecture? Example 2

  17. Proof: Statements Reasons 1. Given 1. ? 2. _____________ 2. Multiplication Prop of Equality 3. 3. Division Property of Equality 4. 4. Symmetric Property of Equality Example 2

  18. A.2A = (b1 + b2)h B. C. D. Example 2

  19. If A B, mB = 2mC, and mC = 45, then mA = 90. Write a two-column proof to verify this conjecture. Write a Geometric Proof Example 3

  20. Proof: Statements Reasons mA = 2(45) 4. Substitution 4. 2.mA = mB 2. Definition of angles A B; mB = 2mC; mC = 45 1. Given 1. 5. mA = 90 5. Arithmetic / Simplification 3. Transitive Property of Equality 3. mA = 2mC Write a Geometric Proof Example 3

  21. Example 3

  22. Proof: 1. Given 1. Statements Reasons ? 2. 2. _______________ 3.AB = RS 3. Definition of congruent segments 4. AB = 12 5. RS = 12 4. Given 5. Substitution Example 3

  23. A. Reflexive Property of Equality B. Symmetric Property of Equality C. Transitive Property of Equality D. Substitution Property of Equality Example 3

  24. Homework Pg 139 (#1-7, 10-20 even)

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