9.2 Proving Quadrilaterals are Parallelograms

1. 9.2 Proving Quadrilaterals are Parallelograms Geometry Mr. Calise

2. Objectives: • Prove that a quadrilateral is a parallelogram. • Use coordinate geometry with parallelograms.

3. Theorem 9-7: If both pairs of opposite sides of a quadrilateral are congruent, then the quadrilateral is a parallelogram. Theorems ABCD is a parallelogram.

4. Statements: AB ≅ CD, AD ≅ CB. AC ≅ AC ∆ABC ≅ ∆CDA BAC ≅ DCA, DAC ≅ BCA AB║CD, AD ║CB. ABCD is a  Reasons: Given Ex. 1: Proof of Theorem 6.6

5. Statements: AB ≅ CD, AD ≅ CB. AC ≅ AC ∆ABC ≅ ∆CDA BAC ≅ DCA, DAC ≅ BCA AB║CD, AD ║CB. ABCD is a  Reasons: Given Reflexive Prop. of Congruence Ex. 1: Proof of Theorem 6.6

6. Statements: AB ≅ CD, AD ≅ CB. AC ≅ AC ∆ABC ≅ ∆CDA BAC ≅ DCA, DAC ≅ BCA AB║CD, AD ║CB. ABCD is a  Reasons: Given Reflexive Prop. of Congruence SSS Congruence Postulate Ex. 1: Proof of Theorem 6.6

7. Statements: AB ≅ CD, AD ≅ CB. AC ≅ AC ∆ABC ≅ ∆CDA BAC ≅ DCA, DAC ≅ BCA AB║CD, AD ║CB. ABCD is a  Reasons: Given Reflexive Prop. of Congruence SSS Congruence Postulate CPCTC Ex. 1: Proof of Theorem 6.6

8. Statements: AB ≅ CD, AD ≅ CB. AC ≅ AC ∆ABC ≅ ∆CDA BAC ≅ DCA, DAC ≅ BCA AB║CD, AD ║CB. ABCD is a  Reasons: Given Reflexive Prop. of Congruence SSS Congruence Postulate CPCTC Alternate Interior s Converse Ex. 1: Proof of Theorem 6.6

9. Statements: AB ≅ CD, AD ≅ CB. AC ≅ AC ∆ABC ≅ ∆CDA BAC ≅ DCA, DAC ≅ BCA AB║CD, AD ║CB. ABCD is a  Reasons: Given Reflexive Prop. of Congruence SSS Congruence Postulate CPCTC Alternate Interior s Converse Def. of a parallelogram. Ex. 1: Proof of Theorem 6.6

10. Theorem 9-8: If both pairs of opposite angles of a quadrilateral are congruent, then the quadrilateral is a parallelogram. Theorems ABCD is a parallelogram.

11. If an angle of a quadrilateral is supplementary to both of its consecutive angles, then the quadrilateral is a parallelogram. Theorems (180 – x)° x° x° ABCD is a parallelogram.

12. Theorem 9-5: If the diagonals of a quadrilateral bisect each other, then the quadrilateral is a parallelogram. Theorems ABCD is a parallelogram.

13. Theorem 9-6: If one pair of opposite sides of a quadrilateral are both congruent and parallel, then the quadrilateral is a parallelogram. Theorems ABCD is a parallelogram.

14. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: 1. Given Ex. 3: Proof of TheoremGiven: BC║DA, BC ≅ DAProve: ABCD is a 

15. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: Given Alt. Int. s Thm. Ex. 3: Proof of Theorem 6.10Given: BC║DA, BC ≅ DAProve: ABCD is a 

16. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: Given Alt. Int. s Thm. Reflexive Property Ex. 3: Proof of Theorem 6.10Given: BC║DA, BC ≅ DAProve: ABCD is a 

17. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: Given Alt. Int. s Thm. Reflexive Property Given Ex. 3: Proof of Theorem 6.10Given: BC║DA, BC ≅ DAProve: ABCD is a 

18. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: Given Alt. Int. s Thm. Reflexive Property Given SAS Congruence Post. Ex. 3: Proof of Theorem 6.10Given: BC║DA, BC ≅ DAProve: ABCD is a 

19. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: Given Alt. Int. s Thm. Reflexive Property Given SAS Congruence Post. CPCTC Ex. 3: Proof of Theorem 6.10Given: BC║DA, BC ≅ DAProve: ABCD is a 

20. Statements: BC ║DA DAC ≅ BCA AC ≅ AC BC ≅ DA ∆BAC ≅ ∆DCA AB ≅ CD ABCD is a  Reasons: Given Alt. Int. s Thm. Reflexive Property Given SAS Congruence Post. CPCTC If opp. sides of a quad. are ≅, then it is a. Ex. 3: Proof of Theorem 6.10Given: BC║DA, BC ≅ DAProve: ABCD is a 

21. Ex. 2: Proving Quadrilaterals are Parallelograms • How do we know these opposite sides are parallel?

22. The 2.75 inch sides of the quadrilateral are opposite and congruent. The 2 inch sides are also opposite and congruent. Because opposite sides of the quadrilateral are congruent, it is a parallelogram. By the definition of a parallelogram, opposite sides are parallel. Ex. 2: Proving Quadrilaterals are Parallelograms

23. Objective 2: Using Coordinate Geometry • When a figure is in the coordinate plane, you can use the Distance Formula to prove that sides are congruent and you can use the slope formula to prove sides are parallel.

24. Show that A(2, -1), B(1, 3), C(6, 5) and D(7,1) are the vertices of a parallelogram. Ex. 4: Using properties of parallelograms

25. Method 1—Show that opposite sides have the same slope, so they are parallel. Slope of AB. 3-(-1) = - 4 1 - 2 Slope of CD. 1 – 5 = - 4 7 – 6 Slope of BC. 5 – 3 = 2 6 - 1 5 Slope of DA. - 1 – 1 = 2 2 - 7 5 AB and CD have the same slope, so they are parallel. Similarly, BC ║ DA. Ex. 4: Using properties of parallelograms Because opposite sides are parallel, ABCD is a parallelogram.

26. Method 2—Show that opposite sides have the same length. AB=√(1 – 2)2 + [3 – (- 1)2] = √17 CD=√(7 – 6)2 + (1 - 5)2 = √17 BC=√(6 – 1)2 + (5 - 3)2 = √29 DA= √(2 – 7)2 + (-1 - 1)2 = √29 AB ≅ CD and BC ≅ DA. Because both pairs of opposites sides are congruent, ABCD is a parallelogram. Ex. 4: Using properties of parallelograms

27. Method 3—Show that one pair of opposite sides is congruent and parallel. Slope of AB = Slope of CD = -4 AB=CD = √17 AB and CD are congruent and parallel, so ABCD is a parallelogram. Ex. 4: Using properties of parallelograms

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