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Holt Geometry

2-2 Conditional Statements2-2 Conditional Statements

Holt Geometry

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Holt Geometry

2-2 Conditional Statements

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Warm UpDetermine if each statement is true or false.

1. The measure of an obtuse angle is less than 90°.

2. All perfect-square numbers are positive.

3. Every prime number is odd.

4. Any three points are coplanar.

F

T

F

T

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Identify, write, and analyze the truth value of conditional statements.

Write the inverse, converse, and contrapositive of a conditional statement.

Objectives

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conditional statementhypothesisconclusiontruth valuenegationconverseinversecontrapostivelogically equivalent statements

Vocabulary

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By phrasing a conjecture as an if-then statement, you can quickly identify its hypothesis and conclusion.

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Identify the hypothesis and conclusion of each conditional.

Example 1: Identifying the Parts of a Conditional Statement

A. If Bessie is a cow then she is a mammal.

B. A number is an integer if it is a natural number.

Hypothesis: Bessie is a cow.

Conclusion: She is a mammal.

Hypothesis: A number is a natural number.

Conclusion: The number is an integer.

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Check It Out! Example 1

"A number is divisible by 3 if it is divisible by 6."

Identify the hypothesis and conclusion of the statement.

Hypothesis: A number is divisible by 6.

Conclusion: A number is divisible by 3.

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“If p, then q” can also be written as “if p, q,” “q, if p,” “p implies q,” and “p only if q.”

Writing Math

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Many sentences without the words if and then can be written as conditionals. To do so, identify the sentence’s hypothesis and conclusion by figuring out which part of the statement depends on the other.

NOTE: Please copy

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Write a conditional statement from the following.

Example 1: Writing a Conditional Statement

The midpoint M of a segment bisects the segment.

If M is the midpoint of a segment, then M bisects the segment.

Identify the hypothesis and the conclusion.

A midpoint, M of a segment

Point M bisects the segment.

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Write a conditional statement from the following.

Example 2: Writing a Conditional Statement

If it is a mosquito, then it is an insect.

The inner oval represents the hypothesis, and the outer oval represents the conclusion.

Mosquito

Insect

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Write a conditional statement from the sentence “Two angles that are complementary are acute.”

If two angles are complementary, then they are acute.

Identify the hypothesis and the conclusion.

Two angles that are complementary

are acute.

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A conditional statement has a truth value of either true (T) or false (F). It is false only when the hypothesis is true and the conclusion is false.

Counterexample: To show that a conditional statement is false, you need to find only one counterexample where the hypothesis is true and the conclusion is false.

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Determine if the conditional is true. If false, give a counterexample.

Example 1: Analyzing the Truth Value of a Conditional Statement

If today is Sunday then tomorrow is Monday.

When the hypothesis is true, the conclusion is also true because Monday follows Sunday. So the conditional is true.

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You can have angles 90°<x<180° to be obtuse. In this case, the hypothesis is true, but the conclusion is false.

If an angle is obtuse, then the angle is 100°.

Since you can find a counterexample, the conditional is false.

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False. Counterexample: Motorcycle

If it has two wheels then it is a bicycle.

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Determine if the conditional “If a number is odd, then it is divisible by 3” is true. If false, give a counterexample.

An example of an odd number is 7. It is not divisible by 3. In this case, the hypothesis is true, but the conclusion is false. Since you can find a counterexample, the conditional is false.

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If the hypothesis is false, the conditional statement is true, regardless of the truth value of the conclusion.

Remember!

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The negation of statement p is “not p,” written as ~p. The negation of a true statement is false, and the negation of a false statement is true.

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Definition Symbols

A conditional is a statement that can be written in the form "If p, then q."

p q

Related Conditionals

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Definition Symbols

The converse is the statement formed by exchanging the hypothesis and conclusion.

q p


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Definition Symbols

The inverse is the statement formed by negating the hypothesis and conclusion.

~p ~q


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Definition Symbols

The contrapositive is the statement formed by both exchanging and negating the hypothesis and conclusion.

~q ~p


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Write the converse, inverse, and contrapositive of the conditional statement. Use the Science Fact to find the truth value of each.

Example 1: Biology Application

If an insect is a butterfly then it has four wings.

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Example 1: Biology Application

Inverse: If an insect is not a butterfly, then it is does not have 4 wings.

Converse: If an insect has 4 wings, then it is a butterfly.

If an insect is a butterfly, then it has four wings.

False. Moth

Contrapositive: If the insect does not have 4 wings, then it is not a butterfly.

Butterflies must have 4 wings. So the contrapositive is true.

False. Moth

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Write the converse, inverse, and contrapostive of the conditional statement “If an animal is a cat, then it has four paws.” Find the truth value of each.


If an animal is a cat, then it has four paws.

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Inverse: If an animal is not a cat, then it does not have 4 paws.

Converse: If an animal has 4 paws, then it is a cat.

Contrapositive: If an animal does not have 4 paws, then it is not a cat; True.

If an animal is a cat, then it has four paws.

There are other animals that have 4 paws that are not cats, so the converse is false.

There are animals that are not cats that have 4 paws, so the inverse is false.

Cats have 4 paws, so the contrapositive is true.

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If it is a fish, then it swims.

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Inverse: If is not a fish, then it does not swim.

Converse: If it swims, then it is a fish.

Contrapositive: If it does not swim, then it is not a fish; True.

If is a fish, then it swims.

False. Counterexample: Human

False. Counterexample: Dog

Fish must swim in water in order to survive.

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If x = 11, then x2 = 121.

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Inverse: If x ≠ 11, then x2 ≠ 121.

Converse: If x2 = 121, then x = 11.

Contrapositive: If x2 ≠ 121, then x ≠ 11; True.

If x = 11, then x2 = 121.

False. Counterexample: x = -11

False. Counterexample: x = -11

Only the square root of 121 will have a result of 11.

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Related conditional statements that have the same truth value are called logically equivalent statements. A conditional and its contrapositive are logically equivalent, and so are the converse and inverse.

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Statement Example Truth Value

Conditional If m<A = 95°, then <A is obtuse.

Converse If <A is obtuse then m<A = 95°.

Inverse If m<A ≠95°, then < A is not obtuse.

Contrapositive If <A is not obtuse, then m<A ≠95°.

T

F

F

T

Note: If the statement is complete true then all the truth values will be true.

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Complete Exit Question

Homework Page 85: # 13-23

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Exit Question: Part I


1. A triangle with one right angle is a right triangle.

2. All even numbers are divisible by 2.

3. Determine if the statement “If n2 = 144, then n = 12” is true. If false, give a counterexample.

H: A number is even. C: The number is divisible by 2.

H: A triangle has one right angle. C: The triangle is a right triangle.

False; n = –12.

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Exit Question: Part II


4. Write the converse, inverse, and contrapositive

of the conditional statement “If Maria’s birthday is

February 29, then she was born in a leap year.”

Find the truth value of each.

Converse: If Maria was born in a leap year, then her birthday is February 29; False. Inverse: If Maria’s birthday is not February 29, then she was not born in a leap year; False. Contrapositive: If Maria was not born in a leap year, then her birthday is not February 29; True.