Building Two Column Proofs
Date: 09/12/98 at 22:00:01 From: Crystal Subject: Geometry - Proving My geometry teacher has just taught us how to do proving, but I don't get it. I know how a certain theorem backs up each statement, but what I don't understand is, how do you know what order to write your statements? Here's an example: <----|----|----|----|----> A B C D Given: AB = CD A-B-C-D Prove: AC = BD Statements Reason AB = CD given BC = BC reflexive property AB + BC = BC + CD addition property of equality A-B-C-D given AC = AB + BC definition of between AC = BC + BD transitive property of equality BC+CD = BD definition of between AC = BD transitive property of equality See, I don't understand how you know what statements to put and in what order, if there is any. Please help.
Date: 09/14/98 at 12:55:26 From: Doctor Peterson Subject: Re: Geometry - Proving Hi, Crystal. A proof is just an orderly way to show that something is true, by building on other things you know are true. The only way that order matters is that each thing you say must be based on something you've already said. Often it will be based on the previous statement, but sometimes you will have to use earlier statements as well. Think of it as building a tower to reach a high goal. Your "givens" are the foundation someone laid for you, and the theorems you have are the girders and rivets you have to put together to make the tower. Let's try drawing your sample proof as a building, to show how its parts are connected. (I've added line numbers.) Statements Reasons 1. AB = CD given 2. BC = BC reflexive property 3. AB + BC = BC + CD addition property of equality 4. A-B-C-D given 5. AC = AB + BC definition of between 6. AC = BC + CD transitive property of equality 7. BC + CD = BD definition of between 8. AC = BD transitive property of equality Here is the foundation at the bottom and the goal at the top: ??AC=BD?? AB=CD A-B-C-D ================= Statement 2 gives me a new fact I can build on that doesn't depend on the givens, but has been applied to a specific value, BC: ??AC=BD?? 2 1 BC=BC AB=CD A-B-C-D ========================= Statement 3 uses statements 1 and 2, by adding them together: ??AC=BD?? 3 AB+BC=BC+CD / \ 2 1 BC=BC AB=CD A-B-C-D ========================= Statement 4 is a given. Statement 5 uses its definition: ??AC=BD?? 3 5 AB+BC=BC+CD AC=AB+BC / \ \ 2 1 4 BC=BC AB=CD A-B-C-D ========================= Statement 6 combines statements 3 and 5 using transitivity: ??AC=BD?? 6 AC=BC+CD / \ 3 5 AB+BC=BC+CD AC=AB+BC / \ \ 2 1 4 BC=BC AB=CD A-B-C-D ========================= Statement 7 is again based on the definition of statement 4: ??AC=BD?? 6 AC=BC+CD / \ 3 5 7 AB+BC=BC+CD AC=AB+BC BC+CD=BD / \ \ / 2 1 4 BC=BC AB=CD A-B-C-D ========================= Now finally we can join statements 6 and 7 to get to the goal AC=BD / \ 6 \ AC=BC+CD \ / \ \ 3 5 7 AB+BC=BC+CD AC=AB+BC BC+CD=BD / \ \ / 2 1 4 BC=BC AB=CD A-B-C-D ========================= Do you see how each statement we put in is supported by some reason (the girder) using one or more previous statements (the rivets)? I could have built some parts in a different order (such as 7 before 5), but nothing can come before what it depends on. We never diagram this way, but it can be very helpful to write after each reason which steps it depends on, if any, like this: Statements Reasons 1. AB=CD given 2. BC=BC reflexive property 3. AB+BC=BC+CD addition property of equality (1,2) 4. A-B-C-D given 5. AC=AB+BC definition of between (4) 6. AC=BC+CD transitive property of equality (3,5) 7. BC+CD=BD definition of between (4) 8. AC=BD transitive property of equality (6,7) I hope that helps. There's a lot more to learn about proofs, like how you can figure out how to get from a given to a goal, but that will come with experience. Write back if you need more ideas. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Date: 09/15/98 at 23:28:23 From: Crystal Lee Subject: Re: Geometry - Proving Dr.Math, Thanks for answering my question. There's one thing I want to make sure. There really isn't any correct order of statements? Crystal
Date: 09/16/98 at 08:58:56 From: Doctor Peterson Subject: Re: Geometry - Proving Hi, again, Crystal. There isn't a single correct order, but often there is a best order, and there are many incorrect orders. In your example, I like the order it is in because most statements follow from the previous statement, possibly combined with an earlier one. But here is another way it could have been done that would be equally valid: Statements Reasons  1. A-B-C-D given  2. AC = AB + BC definition of between (1)  3. BC + CD = BD definition of between (1)  4. AB = CD given  5. BC = BC reflexive property  6. AB + BC = BC + CD addition property of equality (2,5)  7. AC = BC + CD transitive property of equality (2,6)  8. AC = BD transitive property of equality (3,7) Everything still depends only on what came before, so it is a valid proof. Another point to make is that some people prefer to put all the givens as the first steps, rather than saving some for later, as in step 4 here. You will also find that often there are several very different paths you can take to prove something, so that your proof may look very different from someone else's, yet be completely correct. I hope you'll have fun working on proofs. As you should see from this, proofs are really a very creative thing, more like writing (or even art) than the kinds of math you have done before. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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