Section 2-3: Biconditionals & Definitions

Objectives:• Write Biconditionals and recognize good definitions

Conditional Statements and Converses

Statement Example Symbolic You read as

Conditional If an angle is a straight angle, then its measure is 180º.

p q If p, then q.

Converse If the measure of an angle is 180º, then it is a straight angle.

q p If q then p.

Form of a Conditional Statement

• Write a bi-conditional only if BOTH the conditional and the converse are TRUE.

• Connect the conditional & its converse with the word “and”

• Write by joining the two parts of each conditional with the phrase “if and only if” of “iff” for shorthand.

• Symbolically: p q

p q

Bi-conditional Statements

Conditional Statement:If two angles have the same measure, then the angles are congruent.

Converse:If two angles are congruent, then they have the same measure.

Both statements are true, so….

…you can write a Biconditional statement:

Two angles have the same measure if and only if the angles are congruent.

Consider the following true conditional statement. Write its converse. If the converse is also true, combine the statements as a biconditional.

Conditional:

If x = 5, then x + 15 = 20.

Converse:

If x + 15 = 20, then x = 5.

Since both the conditional and its converse are true, you can combine them in a true biconditional using the phrase if and only if.

Biconditional:

x = 5 if and only if x + 15 = 20.

Write a Bi-conditional Statement

Separate a Biconditional

Consider the biconditional statement:A number is divisible by 3 if and only if the sum of its digits is divisible by 3.

• Write a biconditional as two conditionals that are converses of each other.

Statement 1:If a number is divisible by 3, then the sum of its digits is divisible by 3.

Statement 2:If the sum of a numbers digits is divisible by 3, then the number is divisible by 3.

Write the two statements that form this biconditional.

Conditional:

If lines are skew, then they are noncoplanar.

Converse:

If lines are noncoplanar, then they are skew.

Biconditional: Lines are skew if and only if they are noncoplanar.

Separate a Biconditional

Writing Definitions as Biconditionals

Show definition of perpendicular lines is reversible

• Good Definitions: Help identify or classify an object Uses clearly understood terms Is precise avoiding words such as sort of and some Is reversible, meaning you can write a good definition as a biconditional (both

conditional and converse are true)

Definition:Perpendicular lines are two lines that intersect to form right angles

Since both are true converses of each other, the definition can be written as a true biconditional: “Two lines are perpendicular iff they intersect to form right angles.”

Conditional:If two lines are perpendicular, then they intersect to form right angles.

ConverseIf two lines intersect to form right angles, then they are perpendicular.

Show that the definition of triangle is reversible. Then write it as a true biconditional.

Definition: A triangle is a polygon with exactly three sides.

Steps1. Write the conditional2. Write the converse3. Determine if both statements are true4. If true, combine to form a biconditional.

Conditional:If a polygon is a triangle, then it has exactly three sides.Converse:If a polygon has exactly three sides, then it is a triangle.

Biconditional:A polygon is a triangle if and only if it has exactly three sides.

Writing Definitions as Biconditionals

Is the following statement a good definition? Explain.

Conditional: If a fruit is an apple then it contains seeds.

Converse: If a fruit contains seed then it is an apple.

There are many other fruits containing seeds that are not apples, such as lemons and peaches. These are counterexamples, so the reverse of the statement is false.

The original statement is not a good definition because the statement is not reversible.

An apple is a fruit that contains seeds.

Writing Definitions as Biconditionals

Statement Example Symbolic You read as

Conditional If an angle is a straight angle, then its measure is 180º.

p q If p, then q.

Converse If the measure of an angle is 180º, then it is a straight angle

q p If q then p.

Inverse If an angle is not a straight angle, then its measure is not

180.

~p ~q If not p, then not q

Contrapositive If an angle does not measure 180, then the angle is not a

straight angle.

~q ~p If not q, then not p.

Biconditional An angle is a straight angle if and only if its measure is 180º.

p q p if and only if q.

P iff q