Geometric Proof Time!
 Pick up notes from the front table.   
 No Entry Ticket today!
 Tonight’s HW:
o Pg. 113 #1-8
o Pack of notecards by Monday!
 Updates:
o Unit 1 Quiz 4 (2.6, 2.7)
o Monday 9/22
Agenda
1) Warm-Up!
2) 2.6: Proofs
3) Prove THAT!
4) Cool-Down…
Homework Solutions ( p 107)
12. Reflexive Prop of
congruence
30. 3x-1
32. NP  BC
14. Symm Prop of =
26. Reflex Prop of =
28. Reflexive Prop of
congruence
Participation
For days that I do not provide you an entry ticket, I will still be
marking down participation, but on my roster.
I will let you know if you received an X and why when there is
a break in my lesson where I can go to your desk and talk
with you one-on-one.
When we do whiteboards, I do expect 100% participation. If
you do not, I will give you an X on my roster.
Questions?
Warm-Up!
Please take out your whiteboards!
Solve the equation. Write a justification for each step.
1. 5r +2r = -3(r+4)
Learning Objective
By the end of this period you will be able to:
o Write two-column proofs.
Geometric Proof (2.6)
Guess what class….
You have already done proofs!
Now it’s time to do geometric proofs since we are in
Geometry class! 
Geometric Proof (2.6)
Two Column Proof
o The column on the left are the statements, the steps of proving.
o The column on the right are the reasons, the why’s or
justifications.
• The reasons are in the form of a definition, postulate, property, or
theorem.
Geometric Proof (2.5)
You already have this written down on Monday, but lets quickly
review.
Intro to Geometric Proofs
The two questions on the bottom of Monday’s notes are geometric
proofs. Try to fill in the blank!
The first one, I give you the reasons. The next one, you have to use
the properties that we learned in class during the past two class
meetings!
Be ready to share on via the document camera!
Intro to Geometric Proofs
Intro to Geometric Proofs
Geometric Proof (2.6)
Brainstorm!
What definitions,
theorems, postulates,
or properties are we
going to want to
begin thinking
about? Take 1
minute to think
independently.
Write what you
think on your
whiteboard
(1) Practice: Write a justification for each step.
(a) Given that ÐA and ÐB are
supplementary and mÐA = 45°.
Statement
1. ÐA and ÐB are
supplementary.
mÐA = 45°
Reasons
1.
2. mÐA + mÐB = 180°
2.
3. 45° + mÐB = 180°
3.
4. mÐB = 135°
4.
Geometric Proof (2.6)
Brainstorm!
What definitions,
theorems, postulates,
or properties are we
going to want to
begin thinking
about? Now talk
with your table.
Combine
everything on ONE
whiteboard.
(1) Practice: Write a justification for each step.
(a) Given that ÐA and ÐB are
supplementary and mÐA = 45°.
Statement
1. ÐA and ÐB are
supplementary.
mÐA = 45°
Reasons
1.
2. mÐA + mÐB = 180°
2.
3. 45° + mÐB = 180°
3.
4. mÐB = 135°
4.
Geometric Proof (2.6)
Brainstorm!
What definitions,
theorems, postulates,
or properties are we
going to want to
begin thinking
about? Take 1
minute to think
independently.
Write what you
think on your
whiteboard
Geometric Proof (2.6)
Brainstorm!
What definitions,
theorems, postulates,
or properties are we
going to want to
begin thinking
about? Now talk
with your table.
Combine
everything on ONE
whiteboard.
Geometric Proof (2.6)
For practice 2, see if you can fill in the missing pieces of the proof.
Geometric Proof (2.6)
Now, I think you all are ready! I am going to give you cutouts of
two proofs (practice 3 proofs).
The yellow cutouts are the statements.
The green cutouts are the reasons.
See if you and your tablemates can correctly match up the
statements and reasons for both proofs. (You will use each card
only once and when you have completed both proofs you should
have no extra cards.)
When you think you have the proofs correct call me over and I
will check. If they are correct you will copy the proof into practice
3.
Geometric Proof (2.6)
.
Linear Pair Theorem
Here is your first OFFICIAL () Theorem since you all are
EXPERTS!
Since Linear Pair is a Theorem, why don't we prove it!
Linear Pair Theorem Continued…
Given: <1 and <2 are a linear pair.
Prove: <1 and <2 are supplementary.
Hint: There are 6 steps.
What do you think the first step would be?
What do you think the last step would be?
Create a word bank of words you might use to
prove this.
Math Joke of the Day
What do you have to know to get top grades in
geometry?
All the angles!
Geometric Proof (2.6)
You ALREADY proved this in Example 2(b)!!!
GREAT JOB 
Geometric Proof (2.6)
Take out your whiteboard and setup the two columns.
Put your seat number in the top corner of your whiteboard.
Go find someone with the same seat number and switch boards.
On the new board you have write the first step.
Geometric Proof (2.6)
Keep your partners board and go find another partner with the
same seat number and switch boards. Can you set up the second
step? Try your best 
Then do the 3rd step with a different partner.
Geometric Proof (2.6)
Find one more person and have them write the fourth and final
step.
Bring your whiteboard back to your table.
Geometric Proof (2.6)
As a table you are going to analyze all of your proofs.
0 When analyzing you are looking that the proof:
0
0
Makes sense.
Everyone in the group can follow the reasoning.
Pick one persons’ proof at your table and your table will present
their proof to the class using the document camera.
Geometric Proof (2.6)
This proof is very similar to Practice (2b). See if
you and your tablemates can come up with this
complete proof without my help. Good Luck!
Cool-Down…
Table-Share – In your tables answer the following questions.
0
Are you beginning to understand proofs?
0
If you understand proofs, what has helped you understand them?
If you are still struggling, tell your tablemates why you are having
trouble.
Cool-Down…
With the spare time in class I want you to continue writing notecards
or completing your homework.
If you are off task I will ask you to stay in during lunch.