section 2-2 biconditional statements. biconditional statement a statement that contains the phrase...

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Section 2-2 Bicondition al Statements

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Page 1: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Section 2-2Biconditional Statements

Page 2: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Biconditional statement

• a statement that contains the phrase “if and only if”.

• Equivalent to a conditional statement and its converse.

Page 3: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

We can use iff to stand for “If

and only if”

Page 4: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

In order for a biconditional

statement to be TRUE, both the conditional

statement and its converse must be

true.

Page 5: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Example #1:

• Two lines intersect if and only if their intersection is exactly one point.

Write this biconditional statement as a conditional statement.

Page 6: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

• If two lines intersect, then

their intersection is exactly

one point. True

Conditional Statement:

Page 7: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

• If their intersection is exactly one point, then two lines intersect.

Now write the converse.

True

Page 8: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Example #2

• Three lines are coplanar if and only if they lie in the same plane.

Write this biconditional statement as a conditional statement.

Page 9: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

•If three lines are coplanar, then they lie in the same plane. True

Conditional Statement:

Page 10: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

•If three lines lie in the same plane, then they are coplanar.

Now write the converse.

True

Page 11: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

• If an angle is acute then it has a measure between 0° and 90°.

Write the conditional as a biconditional statement.

Page 12: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Write the converse

• If an angle has a measure between 0° and 90°, then it is acute. True

Page 13: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

• 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

Page 14: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Bicondtional:• An angle is acute if and only if it has a measure between 0° and 90°.

Page 15: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

• 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

Page 16: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

Therefore a biconditional for this statement does not exist!

Page 17: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

More Examples:Try It!

Write each conditional as a biconditional statement, if possible. Be sure to give a counterexample if the converse is false!

Page 18: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

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

Page 19: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

• is perpendicular to

iff their intersection forms a right angle.

Biconditional: SR QR

Page 20: Section 2-2 Biconditional Statements. Biconditional statement a statement that contains the phrase “if and only if”. Equivalent to a conditional statement

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: