section 2-2 biconditional statements. biconditional statement a statement that contains the phrase...
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Section 2-2Biconditional Statements
Biconditional statement
• a statement that contains the phrase “if and only if”.
• Equivalent to a conditional statement and its converse.
We can use iff to stand for “If
and only if”
In order for a biconditional
statement to be TRUE, both the conditional
statement and its converse must be
true.
Example #1:
• Two lines intersect if and only if their intersection is exactly one point.
Write this biconditional statement as a conditional statement.
• If two lines intersect, then
their intersection is exactly
one point. True
Conditional Statement:
• If their intersection is exactly one point, then two lines intersect.
Now write the converse.
True
Example #2
• Three lines are coplanar if and only if they lie in the same plane.
Write this biconditional statement as a conditional statement.
•If three lines are coplanar, then they lie in the same plane. True
Conditional Statement:
•If three lines lie in the same plane, then they are coplanar.
Now write the converse.
True
• If an angle is acute then it has a measure between 0° and 90°.
Write the conditional as a biconditional statement.
Write the converse
• If an angle has a measure between 0° and 90°, then it is acute. True
• If it is true, then a biconditional can be written
• If it is false, then a biconditional CAN NOT be written.
Identify whether the converse is true or false
Bicondtional:• An angle is acute if and only if it has a measure between 0° and 90°.
• If an animal is a leopard, then it has spots.
Write the conditional as a biconditional statement.
Write the converse.• If an animal has spots, then
is a leopard. False
Therefore a biconditional for this statement does not exist!
More Examples:Try It!
Write each conditional as a biconditional statement, if possible. Be sure to give a counterexample if the converse is false!
1.If is perpendicular to , then their intersection forms a right angle.SR QR
Converse:
If, and intersect at a right angle, then they are perpendicular to each other.
SR QR
True
• is perpendicular to
iff their intersection forms a right angle.
Biconditional: SR QR
2. If x2 < 49, then x < 7
If x < 7, then x2 < 49.
8x• Counterexample: let then 498 2
Therefore, a biconditional
can not be written!
Converse: