1. 2.2 If - Then Statements OBJ: (1)To Write Statements in If-Then Form
(2)To Write Converse, Inverse, & Contrapositive of Statements
(3)To ID & Use Basic Postulates About Pts, Lines & Planes

2. If - then statements If - then statements are conditional statements
If (hypothesis), then (conclusion)
Hypothesis--- AKA "P" Conclusion--AKA "Q" P Q
The statement
Says If P, then Q or P implies Q

3. Example:
An angle of 40o is acute. Put in If- then form. Identify the hypothesis and the conclusion.
If - then form: If an angle measures 40o, then it is acute.
Conclusion: it is acute
Hypothesis: an angle measures 40o

4. Your Turn:
All triangles have 3 sides. Write in If - then form and identify the hypothesis and conclusion
If a figure is a triangle, then it has 3 sides.
Hypothesis: a figure is a triangle
Conclusion: it has 3 sides.

5. Converse: Change the hypothesis w/ the conclusion.
Example: Adjacent angles have a common side.
Change to if - then form
If 2 angles are adjacent, then they have a common side.
Switch the hypothesis w/ the conclusion
If 2 angles have a common side, then they are adjacent.

6. Biconditional Statements
When a conditional statement and its converse are both true.
We can write it as a single statement
Use the phrase if and only if
Abbreviated with iff
Example: If two lines intersect to form a right angle, then they are perpendicular
If two lines are perpendicular, then they intersect to form a right angle
Two lines are perpendicular if and only if they intersect to form a right angle

7. Example Continued False,
If 2 angles have a common side, then they are adjacent.
Is the converse true or false? If false, provide a counterexample. C D B A
False,
Both angles have a common side, but they are not adjacent because they share interior pts.

8. Your Turn:
Write the converse of the true conditional An angle that measures 120o is obtuse. Determine if the converse is true or false. If false, give a counter example.
Converse: If angle is obtuse, then it measures 120o .
False, because the angle could measure anything > 90o .

9. Negation Denial of a statement.
Example: an angle is obtuse --> an angle is not obtuse
The key to negation is to understand that it is opposite of the original statement. In the example, if the statement is true, then the negation is false. If the statement is false, then the negation is true.

10. Inverse: Original statement: P --> Q If P, then Q
If you are in math class then you are in Mr. Chandler's class.
Inverse statement: not P --> not Q
if not P then not Q
If you are not in math class then you are not in Mr. Chandler's class.

11. Contrapositive: Original statement: p --> Q If p then Q
If you are in math class then you are in Mr. Chandler's class
Contrapositive: not Q --> not P
If not Q then not P
If you are not in Mr. Chandler's class, then you are not in math class.

12. Summary Original: P  Q Inverse: not P  not Q
Converse: Q  P Contrapositive: not Q not P

13. What you need to be able to do:
Be able to convert statements to if - then format
Memorize the 4 if then statement formats
provide counterexamples if statements are false

14. Postulates Principle accepted as true w/o proof 2 pts determine a line
Through 3 nonlinear pts, there is exactly 1 plane
Lines contain at least 2 pts

15. Postulates Continued
A plane contains at least 3 pts not on the same line
if 2 pts lie in a plane, then the line containing the 2 pts lies in the plane
If 2 planes intersect, the intersection is a line

16. Homework:
Put this in your agenda
pg 82 1 – 29 odd,