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Deductive Reasoning

Page history last edited by Andrea Grieser 12 years, 5 months ago

Unlike inductive reasoning, deductive reasoning does not look for patterns and make inferences based on these patterns.  Deductive reasoning uses proven facts, definitions, accepted properties, and the laws of logic to prove conjectures.  Inductive reasoning can help us form conjectures, while deductive reasoning proves them.


Laws of Logic

Some laws of logic used in deductive reasoning are described below:

  • Law of Detachment


If the hypothesis of a conditional statement is true, then the conclusion is true.

For example, given the statement "Mary goes to the movies every Friday and Saturday.  Today is Friday." we can conclude, using the law of detachment, that Mary will go to the movies. 

  • Law of Syllogism
  • ´╗┐The law of syllogism is easiest to illustrate using logic notation.
    If p ->q and q ->r then p -> r
    For example, the statement "If Bob studies his math work he will get an A on the test, and if he gets an A on the test, he will get an A in the course" we can conclude, using the law of syllogism, that if Bob studies his math work he will get an A in the course.

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