Conditional Statments

Warm Up

What is the fourth point of plane XUR

Name the intersection of planes QUV and QTX

Are point U and S collinear?

Quiz

2-1 Conditional Statements

Objectives To recognize conditional statements To write converses of conditional statements

If-Then Statements

Real World Example: “If you are not completely satisfied, then your money will be

refunded.”

Another name of an if-then statement is a conditional. Parts of a Conditional:

Hypothesis (after “If”) Conclusion (after “Then”)

“If you are not completely satisfied, then your money will be refunded.” (hypothesis) (conclusion)

Identifying the Parts

Identify the hypothesis and the conclusion of this conditional statement:

If it is Halloween, then it is October

Hypothesis: It is Halloween Conclusion: It is October

Writing a Conditional

Write each sentence as a conditional: A rectangle has four right angles

“If a figure is a rectangle, then it has four right angles.”

An integer that ends with 0 is divisible by 5

“If an integer ends with 0, then it is divisible by 5.”

Truth Value

A conditional can have a truth value of true or false.

To show that a conditional is true, you must show that every time the hypothesis is true, the conclusion is also true.

To show that a conditional is false, you need to only find one counterexample

Example

Show that this conditional is false by finding a counterexample “If it is February, then there are only 28 days in the

month”

Finding one counterexample will show that this conditional is false

February 2012 is a counterexample because 2012 was a leap year and there were 29 days in February

Converses

The converse of a conditional switches the hypothesis and the conclusion

Example Conditional: “If two lines intersect to form right

angles, then they are perpendicular.”

Converse: “If two lines are perpendicular, then they intersect to form right angles.”

Example

Write the converse of the following conditional: “If two lines are not parallel and do not intersect, then

they are skew”

“If two lines are skew, then they are not parallel and do not intersect.”

Are all converses true?

Write the converse of the following true conditional statement. Then, determine its truth value. Conditional: “If a figure is a square, then it has four

sides”

Converse: “If a figure has four sides, then it is a square”

Is the converse true?

NO! A rectangle that is not a square is a counterexample!

Assessment Prompt

Write the converse of each conditional statement. Determine the truth value of the conditional and its converse.1. If two lines do not intersect, then they are parallel

Converse: “If two lines are parallel, then they do not intersect.”

Conditional is false Converse is true

2. If x = 2, then |x| = 2 Converse: “If |x| = 2, then x = 2”

Conditional is true Converse if false

2-2 Biconditionals

Objectives To write biconditionals

2-2 Biconditionals

When a conditional and its converse are true, you can combine them as a biconditional.

This is a statement you get by connecting the conditional and its converse with the phrase if and only if (iff)

Example of a Biconditional

Conditional If two angles have the same measure, then the angles

are congruent. True

Converse If two angles are congruent, then the angles have the

same measure. True

Biconditional Two angles have the same measure if and only if the

angles are congruent.

Example

Consider this true conditional statement. Write its converse. If the converse is also true, combine them as a biconditional If three points are collinear, then they lie on the same

line.

If three points lie on the same line, then they are collinear.

Three points are collinear if and only if they lie on the same line.

Definitions

A good definition is a statement that can help you identify or classify an object.

A good definition can be written as a biconditional.

Example

Show that this definition of perpendicular lines is a good defintion and that it can be written as a biconditional Definition: Perpendicular lines are two lines that

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

intersect to form right angles. Converse: If two lines intersect to form right angles,

then they are perpendicular. Biconditional: Two lines are perpendicular if and only

if they intersect to form right angles.

Real World Examples

Are the following statements good definitions? Explain An airplane is a vehicle that flies.

Is it reversible? NO! A helicopter is a counterexample because it also

flies!

A triangle has sharp corners. Is it precise? NO! Sharp is an imprecise word!

Homework

Worksheet

QUIZ 2 on MONDAY!