5.6 Indirect Proof

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5.6 Indirect Proof Inequalities in Two Triangles

• Geometry
• Mrs. Spitz
• Fall 2004

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Objectives

• Read and write an indirect proof
• Use the Hinge Theorem and its Converse to compare
  side lengths and angle measures.

3
Assignment

• pp. 305-306 1-24 all

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Using Indirect Proof

• Up to now, all the proofs have used the Laws of
  Syllogism and Detachment to obtain conclusions
  directly. In this lesson, you will study
  indirect proofs. An indirect proof is a proof in
  which you prove that a statement is true by first
  assuming that its opposite is true. If this
  assumption leads to an impossibility, then you
  have proved that the original statement is true.

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Ex. 1 Using Indirect Proof

• Use an indirect proof to prove that a triangle
  cannot have more than one obtuse angle.
• SOLUTION
• Given ? ?ABC
• Prove ??ABC does not have more than one obtuse
  triangle

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Ex. 1 Using Indirect Proof

• Step 1 Begin by assuming that ?ABC does have
  more than one obtuse angle.
• m?A gt 90 and m?B gt 90 Assume ?ABC has two
  obtuse angles.
• m?A m?B gt 180 Add the two given inequalities.
• Step 2 You know however, that the sum of all
  the measures of all three angles is 180.
• m?A m?B m?C 180 Triangle Sum Theorem
• m?A m?B 180 - m?C Subtraction Property of
  Equality

Step 3 So, you can substitute 180 - m?C for
m?A m?B in m?A m?B gt 180 180 - m?C gt 180
Substitution Property of Equality 0 gt m?C
Simplify
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IMPOSSIBLE WHICH IS WHAT WE WANT

• The last statement is not possible angle
  measures in triangles cannot be negative.
• ?So, you can conclude that the original statement
  must be false. That is, ?ABC cannot have more
  than one obtuse triangle.

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Guidelines for writing an Indirect Proof

• Identify the statement that you want to prove is
  false.
• Begin by assuming the statement is false assume
  its opposite is true.
• Obtain statements that logically follow from your
  assumption.
• If you obtain a contradiction, then the original
  statement must be true.

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Objective 2 Using the Hinge Theorem

• In the two triangles shown, notice that AB ? DE
  and BC ? EF, but m?B is greater than m?E.
• It appears that the side opposite the 122 angle
  is longer than the side opposite the 85 angle.
  This relationship is guaranteed by the Hinge
  Theorem.

122
85
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Theorem 5.14 Hinge Theorem

• If two sides of one triangle are congruent to two
  sides of another triangle, and the included angle
  of the first is larger than the included angle of
  the second, then the third side of the first
  triangle is longer than the third side of the
  second triangle.
• RT gt VX

80
100
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Theorem 5.15 Converse of the Hinge Theorem

• If two sides of one triangle are congruent to two
  sides of another triangle, and the third side of
  the first is longer than the third side of the
  second, then the included angle of the first is
  larger than the included angle of the second.
• m?A gt m?D

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Ex. 2 Indirect Proof of Theorem 5.15

• GIVEN AB ? DE
• BC ? EF
• AC gt DF
• PROVE m? B gt m?E
• Solution Begin by assuming that m? gt m?E. Then
  it follows that either m ?B m ?E or m ?B lt m ?E.

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Indirect Proof continued

• Case 1 If m?B m?E, then ?B ? ?E. So, ?ABC ?
  ?DEF by the SAS Congruence Postulate and AC DF.
• Case 2 m?B lt ?E, then AC lt DF by the Hinge
  Theorem.
• Both conclusions contradict the given information
  that AC gt DF. So, the original assumption that
  m?B gt m?E cannot be correct. Therefore, m?B gt
  m?E.

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Ex. 3 Finding Possible Side Lengths and Angle
Measures

• You can use the Hinge Theorem and its converse to
  choose possible side lengths or angle measures
  from a given list.
• AB ? DE, BC ? EF, AC 12 inches, m?B 36, and
  m?E 80. Which of the following is a possible
  length for DF? 8 in., 10 in., 12 in., or 23 in.?

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Ex. 3 Finding Possible Side Lengths and Angle
Measures

• Because the included angle in ?DEF is larger than
  the included angle in ?ABC, the third side DF
  must be longer than AC. So, of the four choices,
  the only possible length for DF is 23 inches. A
  diagram of the triangle shows this is plausible.

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Ex. 3 Finding Possible Side Lengths and Angle
Measures

• ?RST and a ?XYZ, RT ? XZ, ST ? YZ, RS 3.7 cm.,
  XY 4.5 cm, and m?Z 75. Which of the
  following is a possible measure for ?T 60,
  75, 90, or 105.

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Ex. 3 Finding Possible Side Lengths and Angle
Measures

• Because the third side in ?RST is shorter than
  the third side in ?XYZ, the included angle ?T
  must be smaller than ?Z. So, of the four
  choices, the only possible measure for ?T is 60.

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Ex. 4 Comparing Distances

• TRAVEL DISTANCE You and a friend are flying
  separate planes. You leave the airport and fly
  120 miles due west. You then change direction
  and fly W 30 N for 70 miles. (W 30 N indicates
  a north-west direction that is 30 north of due
  west.) Your friend leaves the airport and flies
  120 miles due east. She then changes direction
  and flies E 40 S for 70 miles. Each of you has
  flown 190 miles, but which plane is further from
  the airport?

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SOLUTION

• Begin by drawing a diagram as shown. Your flight
  is represented by ?PQR and your friends flight
  is represented by ?PST.

• Because these two triangles have two sides that
  are congruent, you can apply the Hinge Theorem to
  conclude that RP is longer than TP.
• So, your plane is further from the airport than
  your friends plane.

N
140
W
E
150
S