The Solution (There IS a Better Way!):
So, I added a stage of algebra proofs to fill in the gap that my students were really struggling with. We worked with the typical algebra proofs that are in the book (where students just justify their steps when working with an equation), but then I led them into algebraic proofs that require the transitive property and substitution. We did these for a while until the kids were comfortable with using these properties to combine equations from two previous lines. My "inbetween" proofs for transitioning include multiple given equations (like "Given that g = 2h, g + h = k, and k = m, Prove that m = 3h.") This way, the students can get accustomed to using those tricky combinations of previous lines BEFORE any geometry diagrams are introduced. They are eased into the first Geometry proofs more smoothly. This extra step helped so much. I'll never start Segment and Angle Addition Postulates again until after we've practiced substitution and the transitive property with algebra proofs. Sequencing the Transition:
After finishing my logic unit (conditional statements, deductive reasoning, etc.), I start (as most courses do) with the properties of equality and congruence. I also make sure that everyone is confident with the definitions that we will be using (see the reference list in the download below). I introduce a few basic postulates that will be used as justifications. I spend time practicing with some fun worksheets for properties of equality and congruence and the basic postulates.
Then, when we start twocolumn proof writing, I have students justify basic Algebraic steps using Substitution and the Transitive Property to get the hang of it before ever introducing a diagrambased proof.
This addition made such a difference! By the time the Geometry proofs with diagrams were introduced, the class already knew how to set up a twocolumn proof, develop new equations from the given statements, and combine two previous equations into a new one. Check out this sample proof to see what I'm talking about:
Try It (Download Files):
Taking a couple of days to develop this thought process helped my students so much. After practicing these proofs, they had no problem easing into the next level of proofs with Angle Addition Postulate and Segment Addition Postulate. (Click here for a fun worksheet for practicing with these postulates.) This made them ready for what used to be such a huge leap. We avoided all the struggle that usually comes with introducing proofs. They did not feel nearly as lost.
Try these algebra proofs in your own classroom. You'll love the way this additional lesson leads your students into proof writing more smoothly. This PDF includes a few examples that are halfsheet size. They work really well as warmups.
Pet Peeve to Emphasize:
Here's the other piece the textbooks did not focus on very well  (This drives me nuts). There is a difference between EQUAL and CONGRUENT. This is a mistake I come across all the time when grading proofs. I spend a lot of time emphasizing this before I let my students start writing their own proofs. I make a big fuss over it. I require that converting between the statements is an entire step in the proof, and subtract points if i see something like "<2 = <4" or "<1 + <2 = <3".
When we finally got into the good stuff, after watching me demonstrate a few proofs, a lot of kids would say things like... “Ok I kinda get what you are doing, and each step makes sense, but you are just making it look easy. It seems like you're just making it up." or "I understand some of where it is coming from, but there is just NO WAY I could come up with these steps myself and get from the beginning to the end on my own.” Posters as a Guide When Stuck:
DOWNLOAD POSTERS for FREE: Printable versions of these two pages are included in an email that I send out to subscribers. If you would like to have the 8.5x11 posters for your students, subscribe to the Math Giraffe email list, and they will be sent straight to your inbox!
If you like this sequence and structure for introducing proofs, you may also want to check out my full proof unit or one of these practice packs.
These related resources are available for teaching proofs (Click images to link to products.)
I hope that the downloads I've included will help you organize your proof lessons and get you started with incorporating the new level of algebra proofs. This approach has made a world of difference for me! Let me know how it goes for your students.
To Read Next:
19 Comments
Linda Mitcham
6/22/2015 04:03:32 am
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4/19/2016 10:45:13 pm
Thank you for this great post. I am teaching geometry (which I despised as a student) for the first time, and I appreciate the insights. Can't wait to try this.
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4/20/2016 02:29:06 pm
Hey Doc! :)
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Charles Vochatzer
5/25/2017 08:46:40 pm
I too did not like (despised might work) Geometry in HS. Now, next school year (201718) I get to teach it. Yeah! I'll be looking into getting lots of help with this.
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5/28/2017 04:53:22 pm
Hi Charles,
Terri
8/5/2017 05:23:11 pm
Also first year teaching geo in a long time with new text at a new school, ao looking forward to sharing and learning best practices!
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8/7/2017 02:52:08 pm
Hi Terri,
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Marilyn McArthur
8/7/2018 11:50:29 am
Great ideas. I for the most part have the same students for Algebra I and Geometry  so do try to "prep" them with algebraic proofs during Algebra course. One thing I do require when doing proofs is have also number justification  to correspond with statement. When not using templates they often are hard to match up.
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Becky Z
10/18/2018 09:48:44 pm
I have taught Geometry for the past 3 years, and started my career teaching Geometry from 1996  2003; so I am a veteran of the subject but still change up my technique every year that I teach it. This subtle interlude into proof writing is so clever, and something I haven't tried  I thank you and look forward to doing this lesson before simple Geometry proofs.
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10/22/2018 11:03:14 am
Hi Becky,
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Rebecca
2/12/2019 01:28:46 pm
Using substitution and transitive properties with the algebraic proofs makes so much sense when outlined above. Here's something I can never quite do a good job of explaining. The difference between when to use transitive and when to use substitution. Is one inclusive of the other? Meaning, any transitive property could also be considered substitution property? But not every substitution property is transitive? For example, in the proof just before Try It, could yo have used Substitution based on lines 3 and 5? THanks!
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2/12/2019 02:42:05 pm
Hi Rebecca,
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Chris Beggs
4/19/2019 09:54:33 pm
I agree with your "pet peeve"  I regard it as a "category error", segment AB is in the Geometry category, measure of segment AB is in the Arithmetic (or even Algebra) category.
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4/25/2019 12:11:01 pm
Hi Chris,
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Vicky
10/2/2019 11:52:27 pm
I am trying to download the Proofs Posters guide, but it will not send it to me because I already subscribe to your website. Is there anyway that you can resend those to me?
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10/3/2019 10:22:29 am
Hi Vicky,
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Kal C
5/24/2020 05:44:09 pm
Thanks for this. The style of proofs should be as logical as the proof itself. So I agree with your twocolumn poem structure, which is a systematic way to write a proof. It is essentially the same way that professional mathematics write their proofs, except in prose not in poem.
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Kal C
5/24/2020 05:52:43 pm
Sorry, for Step #5, disregard "using transitivity". That does not apply there.
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5/30/2020 02:31:44 pm
Hi,
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