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Re: Twocolumn Proofs
Posted:
Feb 25, 1993 4:16 PM


DoverSherborn High School (doversherhs@igc.apc.org) wrote: > I am a student of DoverSherborn high, and a member > of Mister Groves sophmore geometry class and we have done > equal numbers of two column proofs and paragraph style > proofs, and I have found paragraph style the most usefull. > The paragraph style encourages us to write down all we > know, but the two column proof only needs or asks for a > certain thing....
I was delighted to read your response. It seems that you are learning a lot about mathematics in your geometry class. However let me address some points about the nature of proving which your article indirectly raises.
There are (at least) two aspects of proving a theorem. The first is finding a proof; the second is writing it down in order to communicate it. To use a weak analogy, the first part is similar to searching for something you lost. In the course of your search you come across many things some of which are insignificant. These insignificant things you may put back where they belong. Hopefully you will finally find what you seek. In searching for a proof, you write down possibly insignificant facts and look for some connection among them which will lead to a proof. Again hopefully you succeed.
The second part of proving is to write the proof clearly. When you write the proof, you should leave out all of the ideas which you did not use. These will only distract and confuse the reader. You should communicate only those ideas which demonstrate why a theorem is true. This second part if often misunderstood by those just learning to write proofs (even in college!).
If you will allow me to be a little bit critical, it seems to me that your conception of a twocolumn proof is to do the second part without doing the first part. And your conception of the paragraph style is to do only the first part. It seems to me that after you write your first "paragraph proof", you should rewrite and leave out the insignificant stuff. In fact when my students include unnecessary statements in their proofs, I point it out.
Let me clearly say that I do think that you should begin by writing down everything that seems relevant. What you describe yourself doing for a "paragraph style" proof is an excellent way to discover a proof. Every proof requires at least two sheets of paperone for finding the proof and one for writing it up.
The twocolumn "method" focuses one's attention on the second phase of proving a theorem. It forces one to clearly show why your assertions are true. One should also be forced to do this in the paragraph style. One problem with the twocolumn style is that it focuses so much attention on the second part of proving, the writeup, that students do not realize that they are supposed to still do the first part. And as you said, without the first part, you are likely to fail to find a proof.



