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16. 5x – 3 = 4(x + 2) Given 17. 1.6 = 3.2n Given

16. 5x – 3 = 4(x + 2) Given 17. 1.6 = 3.2n Given 5x – 3 = 4x + 8 Dist. Prop 0.5 = n Division x – 3 = 8 Subt. x = 11 Subt. 23.  Add Post; Subst.; Simplify; Subtr.; Add; Division

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16. 5x – 3 = 4(x + 2) Given 17. 1.6 = 3.2n Given

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  1. 16. 5x – 3 = 4(x + 2) Given 17. 1.6 = 3.2n Given 5x – 3 = 4x + 8 Dist. Prop 0.5 = n Division x – 3 = 8 Subt. x = 11 Subt. 23.  Add Post; Subst.; Simplify; Subtr.; Add; Division 24.  Add Post; Subst.; Dist. Prop; Simplify; Subtr; Division 25. Sym Prop 34. 169.50 = 35 + 21(3) + 1.10x Given 26. Reflex. Prop 169.50 = 98 + 1.10x Simplify 27. Trans. Prop 71.50 = 1.10x Subtr. 28. Reflex. Prop 65 = x Division 30. 3x – 1 39. B 31. A  T 40. H 32. NP  BC 41. D 33. x = 5, y = 9 42. 90

  2. Warm Up Determine whether each statement is true or false. If false, give a counterexample. 1.It two angles are complementary, then they are not congruent. 2. If two angles are congruent to the same angle, then they are congruent to each other. 3. Supplementary angles are congruent. false; 45° and 45° true false; 60° and 120°

  3. Definitions • Postulates • Properties • Theorems Conclusion Hypothesis When writing a proof, it is important to justify each logical step with a reason. You can use symbols and abbreviations, but they must be clear enough so that anyone who reads your proof will understand them. Example 1: Write a justification for each step, given that A and Bare supplementary and mA = 45°. 1. Aand Bare supplementary. mA = 45° Given information Def. of supp s 2. mA+ mB= 180° 3. 45°+ mB= 180° Subst. Prop of = Steps 1, 2 4. mB= 135° Subtr. Prop of =

  4. A theorem is any statement that you can prove. Once you have proven a theorem, you can use it as a reason in later proofs.

  5. A geometric proof begins with Given and Prove statements, which restate the hypothesis and conclusion of the conjecture. In a two-column proof, you list the steps of the proof in the left column. You write the matching reason for each step in the right column. Example 2: Fill in the blanks to complete the two-column proof. Given: XY Prove: XY  XY Reflex. Prop. of =

  6. Helpful Hint If a diagram for a proof is not provided, draw your own and mark the given information on it. But do not mark the information in the Prove statement on it. Before you start writing a proof, you should plan out your logic. Sometimes you will be given a plan for a more challenging proof. This plan will detail the major steps of the proof for you.

  7. Example 3: Use the given plan to write a two-column proof. Given: 1 and 2 are supplementary, and 1  3 Prove: 3 and 2 are supplementary. Plan: Use the definitions of supplementary and congruent angles and substitution to show that m3 + m2 = 180°.By the definition of supplementary angles, 3 and 2are supplementary. 1 and 2 are supplementary. 1  3 Given m1+ m2 = 180° Def. of supp. s m1= m3 Def. of s Subst. m3+ m2 = 180° 3 and 2 are supplementary Def. of supp. s

  8. Example 3 Use the given plan to write a two-column proof if one case of Congruent Complements Theorem. Given: 1 and 2 are complementary, and 2 and 3 are complementary. Prove: 1  3 1 and 2 are complementary. 2 and 3 are complementary. Given Def. of comp. s m1+ m2 = 90° m2+ m3 = 90° m1+ m2 = m2+ m3 Subst. m2= m2 Reflex. m1 = m3 Subtr. 1  3 Def. of  s

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