5'3 Indirect Proof

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5.3 Indirect Proof
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Types of Reasoning

• Direct Reasoning
• Law of detach
• Law of syll
• Direct proof
• 2-column (formal)
• Paragraph (informal)

• Indirect Reasoning
• Indirect proof (proof by contradiction)

Start w/ assuming opp of prove justify ea.
step to lead to a contradiction of a given or
some other fact. (in parragraph form)
Start w/ given justify ea. step to end w/
whats to be proved.
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Steps for Writing an Indirect Proof

• Assume concl (prove) is false.
• Show that the assumption leads to a contradiction
  of the hypothesis or some other fact, such as a
  post, thm, or cor.
• Point out that the assumption must be false, ,
  the concl must be true.

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Ex Write an Indirect Proof
E
A
C
D
B
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Ex Give the assumption to start an indirect
proof

• AB ? MN
• ?ABC is isosc
• x 4
• x gt -27
• If 9 is a factor of n ?3 is a factor of n.

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Ex Prove indirectly

• Given 2x-3 gt 7
• Prove x gt 5

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Ex Prove Indirectly
1
l
3
m
2
8

• 5.3 worksheet