Logic

A statement is a sentence that is either true or false (its truth value).

Logically speaking, a statement is either true or false. What are the values of these statements? The sun is hot. The moon is made of cheese. A triangle has three sides. The area of a circle is 2πr.

Statements can be joined together in various ways to make new statements.

Conditional Statements A conditional (or propositional) statement has two parts:

A hypothesis (or condition, or premise) A conclusion (or result)

Many conditional statements are in “If… then…” form. Ex.: If it is raining outside, then I will get wet.

A conditional statement is made of two separate statements; each part has a truth value. But the overall statement has a separate truth value. What are the values of the following statements? If today is Friday, then tomorrow is Saturday. If the sun explodes, then we can live on the moon. If a figure has four sides, then it is a square.

Conditional Statements

Conditional statements don’t have to be “If… then…” See if you can determine the condition and conclusion in each of the following, and restate in “If… then…” form.An apple a day keeps the doctor away.What goes up must come down.All dogs go to heaven.Triangles have three sides.

Inverse

The inverse of a statement is formed by negating both its hypothesis and conclusion.

Statement:If I take out my cell phone, then Mr. Peterson

will confiscate it. Inverse:

If I do take out my cell phone, then Mr. Peterson will confiscate it.

notnot

Try these

Give the inverses for the following statements. (You may wish to rewrite as “If… then…” first.) Then determine the truth value of the inverse.Barking dogs give me a headache.If lines are parallel, they will not intersect.I can use the Pythagorean Theorem on right

triangles.A square is a four-sided figure.

Converse

A statement’s converse will switch its hypothesis and conclusion.

Statement:If I am happy, then I smile.

Converse:If , then .I smileI am happy

Try these

Give the converses for the following statements. Then determine the truth value of the converse.If I am a horse, then I have four legs.When I’m thirsty, I drink water.All rectangles have four right angles.If a triangle is isosceles, then two of its sides

are the same.

ContrapositiveA contrapositive is a combination of a

converse and an inverse. The premise and conclusion switch, and both are negated.

Statement:If my alarm has gone off,

then I am awake.Contrapositive:

If ,then .I am not awake

my alarm has not gone off not

not

Try these Give the contrapositives for the following

statements. Then determine its truth value.If it quacks, then it is a duck.When Superman touches kryptonite, he gets

sick.If two figures are congruent, they have the

same shape and size.A pentagon has five sides.

Note: A contrapositive always has the same truth value as the original statement!

Symbolic representation Logic is an area of study, related to math (and

computer science and other fields). In formal logic, we can represent statements symbolically (using symbols).

Some common symbols:

a statement, usually a premise

a statement, usually a conclusion

creates a conditional statement

negates a statement (takes its opposite)

q

p

or

or

~

Examples

If p, then q Inverse:

If not p, then not q Converse:

If q, then p Contrapositive

If not q, then not p

qp

qp ~~

pq

pq ~~

Truth Table

A truth table is a way to organize the truth values of various statements.In a truth table, the columns are statements

and the rows are possible scenarios.The table contains every possible scenario

and the truth values that would occur. Example: p ~p

T

TF

F

A conditional truth table

p q p → q

T T

T F

F T

F F

T

F

T

T

A conditional truth table

p q p → q

T T

T F

F T

F F

T

F

T

T

q → p ~p →~q ~q →~p

T

T

F

T

T

T

F

T

T

F

T

T

Two statements are considered logical equivalents if they have the same truth value in all scenarios. A way to determine this is if all the values are the same in every row in a truth table.

Logical Equivalents

Logical Equivalents

Which of the following statements are logically equivalent?

p q p → q

T T

T F

F T

F F

T

F

T

T

q → p ~p →~q ~q →~p

T

T

F

T

T

T

F

T

T

F

T

T

Conjunctions A conjunction consists of two statements

connected by ‘and’. Example:

Water is wet and the sky is blue. Notation:

A conjunction of p and q is written as qp

Conjunctions A conjunction is true only if

both statements are true.

p q p ^ q

T T

T

T

T

F

F

F F

F

F

F

Remember: the truth value of a conjunction refers to the statement as a whole.

Consider: “The sun is out and it is raining.”

Disjunctions A disjunction consists of two statements

connected by ‘or’. Example:

I can study or I can watch TV. Notation:

A disjunction of p and q is written as qp

Disjunctions A disjunction is true if either

statement is true.

p q p v q

T T

T

T

T

F

F

F F

T

T

F

Consider: “Timmy goes to Stanton or he goes to Paxon.”

Homework

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