# Geometric proof: term for "it is wrong" **URL:** \ **Category:** Factual Questions\ **Created:** [March 1, 2019, 4:04pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381 "2019-03-01T16:04:44Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [March 1, 2019, 4:04pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/1 "2019-03-01T16:04:44Z")
I have a distant memory that in geometric proofs, there was a term that the teacher could insert at the end if the result has an error. Essentially saying, “result is absurd so there must be a mistake; re-do” Opposite of “Q.E.D.” Does this ring a bell with anyone? ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [March 1, 2019, 4:10pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/2 "2019-03-01T16:10:28Z")
According to [this,](https://www.tes.com/teaching-resource/mathematical-and-numeracy-marking-codes-11366036) a “??” ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [March 1, 2019, 4:12pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/3 "2019-03-01T16:12:03Z")
False? Contradictory? Absurd? Bogus? Wrong? Note that QED does not mean “true” or “correct”. Was it a Latin phrase you were looking for? ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [March 1, 2019, 4:29pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/4 "2019-03-01T16:29:51Z")
> [@Northern\_Piper](#): > > I have a distant memory that in geometric proofs, there was a term that the teacher could insert at the end if the result has an error. > > Essentially saying, “result is absurd so there must be a mistake; re-do” > > Opposite of “Q.E.D.” “W.T.F.”? 😉 ---
**Author:** ![Peter\_Morris](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/peter_morris/32/359_2.png) [@Peter\_Morris](https://boards.straightdope.com/u/Peter_Morris)\ **Post date:** [March 1, 2019, 4:35pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/5 "2019-03-01T16:35:10Z")
[Reductio ad absurdum](https://en.wikipedia.org/wiki/Reductio_ad_absurdum) ---
**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\ **Post date:** [March 1, 2019, 6:41pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/6 "2019-03-01T18:41:56Z")
Probably not what you’re looking for but traditionally in Euclidian geometry, Q.E.D. (_quod erat demonstrandum_ = “what was to be demonstrated”) is used at the end of the proof of a theorem. Q.E.F. (_quod erat faciendum_ = “what was to be done”) is used at the end of a problem or exercise that doesn’t involve proving a theorem. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [March 1, 2019, 8:25pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/7 "2019-03-01T20:25:44Z")
Well, I taught math for nearly 40 years and, while such a term would have been all too useful, I never knew it. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [March 1, 2019, 8:32pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/8 "2019-03-01T20:32:49Z")
One of my profs used “Badly Substantiated” or “BS” for short. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [March 1, 2019, 9:22pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/9 "2019-03-01T21:22:45Z")
Non Sequitur - “it does not follow” Or perhaps - **Proof by contradiction**. In logic, proof by contradiction is a form of proof that establishes the truth or validity of a proposition by first assuming that the opposite proposition is true, and then shows that such an assumption leads to a contradiction. _reductio ad impossibilem._ ---
**Author:** ![mbh](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@mbh](https://boards.straightdope.com/u/mbh)\ **Post date:** [March 1, 2019, 9:58pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/10 "2019-03-01T21:58:12Z")
“I think you should be more explicit here in step two.” > **[Science Cartoons Plus -- The Cartoons of S. Harris](http://www.sciencecartoonsplus.com/pages/gallery.php)** > > The cartoons of S. Harris ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [March 2, 2019, 12:39am UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/11 "2019-03-02T00:39:13Z")
> [@md2000](#): > > Non Sequitur - “it does not follow” > > Or perhaps - > > **Proof by contradiction**. In logic, proof by contradiction is a form of proof that establishes the truth or validity of a proposition by first assuming that the opposite proposition is true, and then shows that such an assumption leads to a contradiction. _reductio ad impossibilem._ That’s it! Isaac Barrow, one of Isaac Newton’s teachers, would end such a proof with “Quod est absurdum” - “Q.E.A.” See the section on “notation” in the wiki article: > **[Proof by contradiction](https://en.wikipedia.org/wiki/Proof_by_contradiction)** > > In logic, proof by contradiction is a form of proof that establishes the truth or the validity of a proposition, by showing that assuming the proposition to be false leads to a contradiction. > Although it is quite freely used in mathematical proofs, not every school of mathematical thought accepts this kind of nonconstructive proof as universally valid. > More broadly, proof by contradiction is any form of argument that establishes a statement by arriving at a contradiction, even when the initial... ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [March 2, 2019, 5:34am UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/12 "2019-03-02T05:34:41Z")
AFQ. From a false proposition, anything follows. Or you can make up your own 🙂 QFE: quod falsum est. “Which is false” ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [March 2, 2019, 9:48pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/13 "2019-03-02T21:48:56Z")
If you are writing intelligently, you would state that the goal is to get a contradiction if the claim is false: Step 1. Assuming X is false, we will show that it leads to a contradiction. Steps 2 … n-1 (various stuff) Step n. And therefore we see that 0=1 which is the contradiction we sought. Q.E.D. Some text as to why the contradiction is aimed for is helpful so there would be no need to end on a negative conclusion. You phrase it as a positive goal. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [March 2, 2019, 11:51pm UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/14 "2019-03-02T23:51:27Z")
quod errata demonstrandum ---
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [March 3, 2019, 12:17am UTC](https://boards.straightdope.com/t/geometric-proof-term-for-it-is-wrong/830381/15 "2019-03-03T00:17:04Z")
When my father was teaching English, he had a rubber stamp made up that he would use to indicate that a statement made is blatantly false and needs to be rewritten. “Piffle.”