2.3: Converse, Inverse, and Contrapositive
What if your sister told you " if you do the dishes, then I will help you with your homework "? What's a statement that is logically equivalent to what your sister said? After completing this Concept, you will know how to answer this question as you discover converses, inverses, and contrapositives, and how changing a conditional statement affects truth value.
Watch This
CK12 Foundation: Chapter2ConverseInverseContrapositiveA
James Sousa: Converse, Contrapositive, and Inverse of an IfThen Statement
Guidance
Consider the statement: If the weather is nice, then I will wash the car. This can be rewritten using letters to represent the hypothesis and conclusion:
In addition to these positives, we can also write the negations, or “not”s of and . The symbolic version of not , is .
Using these negations and switching the order of and , we can create three more conditional statements.
If we accept “If the weather is nice, then I’ll wash the car” as true, then the converse and inverse are not necessarily true. However, if we take the original statement to be true, then the contrapositive is also true. We say that the contrapositive is logically equivalent to the original ifthen statement. It is sometimes the case that a statement and its converse will both be true. These types of statements are called biconditional statements. So, is true and is true. It is written , with a double arrow to indicate that it does not matter if or is first. It is said, “ if and only if ”. Replace the “ifthen” with “if and only if” in the middle of the statement. “If and only if” can be abbreviated “iff.”
Example A
Use the statement: If , then .
a) Find the converse, inverse, and contrapositive.
b) Determine if the statements from part a are true or false. If they are false, find a counterexample.
The original statement is true.
Example B
Use the statement: If I am at Disneyland, then I am in California.
a) Find the converse, inverse, and contrapositive.
b) Determine if the statements from part a are true or false. If they are false, find a counterexample.
The original statement is true.
Notice for the inverse and converse we can use the same counterexample. This is because the inverse and converse are also logically equivalent.
Example C
The following is a true statement:
if and only if is an obtuse angle.
Determine the two true statements within this biconditional.
Statement 1 : If , then is an obtuse angle
Statement 2: If is an obtuse angle, then .
You should recognize this as the definition of an obtuse angle. All geometric definitions are biconditional statements.
Watch this video for help with the Examples above.
CK12 Foundation: Chapter2ConverseInverseContrapositiveB
Concept Problem Revisited
Your sister presented you with the ifthen statement, " If you do the dishes, then I will help you with your homework ." If we take the original statement to be true, then the contrapositive is also true. The following contrapositive statement is logically equivalent to the original ifthen statement:
" If I do not help you with your homework, then you will not do the dishes ."
Guided Practice
1. Use the statement: Any two points are collinear.
a) Find the converse, inverse, and contrapositive.
b) Determine if the statements from part a are true or false. If they are false, find a counterexample.
2.
a) Is true? If not, find a counterexample.
b) Is true? If not, find a counterexample.
c) Is true? If not, find a counterexample.
d) Is true? If not, find a counterexample.
Answers:
1. First, change the statement into an “ifthen” statement: If two points are on the same line, then they are collinear.
2.
Explore More
For questions 14, use the statement: If and , then is the midpoint of .
 If this is the converse, what is the original statement? Is it true?
 If this is the original statement, what is the inverse? Is it true?
 Find a counterexample of the statement.
 Find the contrapositive of the original statement from #1.
 What is the inverse of the inverse of ? HINT: Two wrongs make a right in math!
 What is the oneword name for the converse of the inverse of an ifthen statement?
 What is the oneword name for the inverse of the converse of an ifthen statement?
 What is the contrapositive of the contrapositive of an ifthen statement?
For questions 912, determine the two true conditional statements from the given biconditional statements.
 A U.S. citizen can vote if and only if he or she is 18 or more years old.
 A whole number is prime if and only if it has exactly two distinct factors.
 Points are collinear if and only if there is a line that contains the points.
 if and only if .

 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.

 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.

the measure of
is a right angle
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.

the measure of
is an acute angle
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Is true? If not, find a counterexample.
 Write a conditional statement. Write the converse, inverse and contrapositive of your statement. Are they true or false? If they are false, write a counterexample.
 Write a true biconditional statement. Separate it into the two true conditional statements.
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Description
Learning Objectives
Here you'll learn how to find the converse, inverse and contrapositive of a conditional statement. You will also learn how to determine whether or not a statement is biconditional.