# Sec.2-3 Deductive Reasoning

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Sec.2-3 Deductive Reasoning
Objective: a) To use the Law of Detachment. b) To use the Law of Syllogism.

In Ch. 1 we used inductive reasoning to predict what will happen next
In Ch.1 we used inductive reasoning to predict what will happen next. Inductive reasoning is based on patterns you observe. Today we are going to look at the other kind of reasoning. Read

I. Deductive Reasoning AKA….Logical reasoning
System for reaching logical conclusions Cornerstone of geometry (Proofs) Start w/ a given true statement, then state the next logical statement based on the given. Read

Deductive reasoning is the process of using logic to draw conclusions from given facts, definitions, and properties. Write

II. Law of Detachment In deductive reasoning, if the given facts are true and you apply the correct logic, then the conclusion must be true. The Law of Detachment is one valid form of deductive reasoning. Read Law of Detachment If p  q is a true statement and p is true, then q is true. Write

Example 1: Using the Law of Detachment
Given: I know if it is raining at my house, then water is being added to my pool. Statement: It is raining at my house What conclusion: Water is being added to my pool.

Example 2B: Verifying Conjectures by Using the Law of Detachment
Determine if the conjecture is valid by the Law of Detachment. Given: In the World Series, if a team wins four games, then the team wins the series. The Red Sox won four games in the 2004 World Series. Conjecture: The Red Sox won the 2004 World Series.

Example 2B: Verifying Conjectures by Using the Law of Detachment
Identify the hypothesis and conclusion in the given conditional. In the World Series, if a team wins four games, then the team wins the series. The statement “The Red Sox won four games in the 2004 World Series” matches the hypothesis of a true conditional. By the Law of Detachment, the Red Sox won the 2004 World Series. The conjecture is valid.

III. Law of Syllogism Another valid form of deductive reasoning is the Law of Syllogism. It allows you to draw conclusions from two conditional statements when the conclusion of one is the hypothesis of the other. Read Law of Syllogism If p  q and q  r are true statements, then p  r is a true statement. Write

Example: Verifying Conjectures by Using the Law of Syllogism
Determine if the conjecture is valid by the Law of Syllogism. Given: If a figure is a kite, then it is a quadrilateral. If a figure is a quadrilateral, then it is a polygon. Conjecture: If a figure is a kite, then it is a polygon.

Example 3A: Verifying Conjectures by Using the Law of Syllogism Continued
Let p, q, and r represent the following. p: A figure is a kite. q: A figure is a quadrilateral. r: A figure is a polygon. You are given that p  q and q  r. Since q is the conclusion of the first conditional and the hypothesis of the second conditional, you can conclude that p  r. The conjecture is valid by Law of Syllogism.

Determine if the conjecture is valid by the Law of Syllogism.
Example Determine if the conjecture is valid by the Law of Syllogism. Given: If an animal is a mammal, then it has hair. If an animal is a dog, then it is a mammal. Conjecture: If an animal is a dog, then it has hair. Is the conclusion of one statement the conclusion of the other? If so, you can use the law of syllogism Y can skip q and go straignt to p

Example Continued Let x, y, and z represent the following. x: An animal is a mammal. y: An animal has hair. z: An animal is a dog. You are given that x  y and z  x. Since x is the conclusion of the second conditional and the hypothesis of the first conditional, you can conclude that z  y. The conjecture is valid by Law of Syllogism.

Example 2: Given 1: If two planes intersect, then they intersect in a line. Given 2: If two planes are not parallel, then they intersect. Statement: Plane ABC and Plane XYZ are not parallel. Conclusion: The planes intersection is a line.

What did I learn today? If a figure is a rectangle, then it has two pairs of parallel sides. Figure ABCD is a rectangle. Conclusion: ABCD has 2 pairs of parallel sides. Which law did you use to come to your conclusion? Law of Detachment

What is your conclusion and what law did you use?
If you live in Indianapolis, then you live in Indiana. Larry lives in Indy. If you live in Indiana then you live north of the Ohio River. Conclusion? Larry lives north of the Ohio River. Which law(s) Both