Geometry Honors Section 2.2 Introduction to Logic and Introduction to Deductive Reasoning

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Geometry Honors Section 2.2Introduction to Logic and Introduction to Deductive Reasoning

whale

mammal

whale

mammal

The figures at the right are Venn diagrams. Venn diagrams are also called __________ diagrams, after the Swiss mathematician _______________________.Which of the two diagrams correctly represents the statement“If an animal is a whale, then it is a mammal”.

Euler

Leonard Euler

If-then statements like statement (1) are called *___________ In a conditional statement, the phrase following the word “if” is the *_________. The phrase following the word “then” is the *_________.

conditionals.

hypothesis

conclusion

If you interchange the hypothesis and the conclusion of a conditional, you get the *converseof the original conditional.

Example 1: Write a conditional statement with the hypothesis “an animal is a reptile” and the conclusion “the animal is a snake”. Is the statement true or false? If false, provide a counterexample.

If an animal is a reptile than it is a snake.

False

Write the converse of the conditional statement. Is the statement true or false? If false, provide a counterexample.

If an animal is a snake, then it is a reptile.

True

Example 2: Consider the conditional statement “If two lines are perpendicular, then they intersect to form a right angle”. Is the statement true or false? If false, provide a counterexample.

Write the converse of the conditional statement. Is the statement true or false? If false, provide a counterexample.

When an if-then statement and its converse are both true, we can combine the two statements into a single statement using the phrase “if and only if” which is often abbreviated iff.Example: Combine the two statements in example 2, into a single statement using iff.

Reasoning based on observing patterns, as we did in the first section of Unit I, is called inductivereasoning. A serious drawback with this type of reasoning is

your conclusion is not always true.

logically correct conclusions

always give a correct conclusion.

We will reason deductively by doing two column proofs. In the left hand column, we will have statements which lead from the given information to the conclusion which we are proving. In the right hand column, we give a reason why each statement is true. Since we list the given information first, our first reason will always be ______. Any other reason must be a _________, _________ or ________.

given

theorem

postulate

definition

Our first proofs will be algebraic proofs. Thus, we need to review some algebraic properties. These properties, like postulates are accepted as true without proof.

a= a

If a = b, then b = a

Addition Property of Equality:Subtraction Property of Equality:Multiplication Property of Equality:

If a = b, then a+c=b+c

If a = b, then a-c = b-c

If a = b, then ac =bc

Substitution Property:

If two quantities are equal, then one may be substituted for the other in any equation or inequality.

Example: Complete this proof:

Given

Multiplication Property

Distributive Property