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This question comes out of curiosity. I have seen some people mention you just divided by zero. For example,

These two cases may or may not be in different contexts, but is there any meaning to the expression? I guess it means you did a difficult task. Is that true?

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It can be used to mean you just did the impossible. It is also thought that it can have catastrophic effects. (See what happened when a ship's computer tried to divide by zero in this entry: en.wikipedia.org/wiki/Division_by_zero ) –  JLG Apr 17 '12 at 11:27
    
@JLG, thanks. I got it. –  gt_ebuddy Apr 17 '12 at 11:35
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Dividing by zero isn't "difficult"; it's impossible (undefined). And forbidden. Generally people like to make jokes about dividing by zero doing insert bad thing here (creating black holes seems to be popular), just like typing google into google supposedly breaks the internet (which is just a hoax).

So I guess it's just a way of saying "you shouldn't have done that," or "you screwed up badly".

Edit: For completeness sake, I'll add the information given by T.E.D. in one of the other answers here. Kudos to him for this.

Aside from being impossible, dividing by zero could be used to create proofs of things that are clearly false. For example, one could prove that 2 + 2 = 5 when allowed to divide by zero. An example of this ("proving" 1 = 2)

Taking this into account one could state that saying someone just "divided by zero" means someone did something that's invalid (in, for example, a mathematical proof)

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This answer misses the nuance (found in the first example at least) that this is often done on purpose to mislead people. –  T.E.D. Apr 17 '12 at 16:03
    
Indeed it does. Yours did though, so I didn't feel the immediate need to add this. Should I? I'm not sure what the policy is on this, but it feels kind of cheap to copy your (better/more complete in that aspect) answer to mine... –  MaienM Apr 17 '12 at 16:13
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A damn good question. Personally, sometimes I do that, sometimes it doesn't seem right. In this case, as its the accepted answer (with the assumption that the acceptance isn't likely to be revoked), I kind of lean toward having you add it. Many people don't read past an accepted answer. (This is why question posters should wait more than 1 hour before accepting an answer) –  T.E.D. Apr 17 '12 at 16:28
    
Alright then, I will add it. –  MaienM Apr 17 '12 at 17:05
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I use this phrasing myself a lot. Sadly, most folks without a math background don't understand. It is true that in mathematics dividing by zero is (generally) not allowed. There's more to it than that though.

There are rather a lot of semi-famous mathematical proofs that "prove" clearly insane things (e.g.: 1=2) by sneaking in a divide by zero as one of the steps. Essentially, you can prove anything, if allowed to divide by zero.

So to rhetorically say that someone has "divided by zero" is to say that they just inserted something flat out wrong in their logic, generally for the purpose of being able to "prove" something else that is also flat out wrong (or at least otherwise difficult to prove).

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When you divide by zero on a handheld calculator, the result is usually a display of "error" or "undefined":
enter image description here enter image description here

When you try dividing by zero on the Window 7 calculator, you are informed it can't be done: enter image description here

In Excel, you get a Divide-by-Zero error:
enter image description here

And when it happens in a computer program, a divide-by-zero exception is raised. The software should be designed to handle that, but that's not always the case. When precautionary programming measures aren't taken, the calculation can cause an unexpected computer crash. (The most notable incident was when a divide-by-zero error crippled a U.S. Navy vessel for over two hours; this is frequently cited as among the most notorious software bugs of all time, and is ranked #7 on this Top 10 list).

With all that as a backdrop, software engineers sometimes use the phrase "you just divided by zero" to be a very polite euphemism for "Oh, s∗∗∗; you just f∗∗∗∗∗ up..."

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Its QA site about English language usage. And to be clear, I am not asking about Divide By Zero exceptions in computer program and calculators. Anyway thanks for the great effort you made in answering my question. –  gt_ebuddy Apr 17 '12 at 15:46
    
@gt_ebuddy: I've attempted to answer your question from an English usage perspective, even if the preface of my answer was a bit long-winded. There's a reason this expression is used the way that it is, and that reason is rooted in how computers handle the divide-by-zero problem. Anyhow, thanks for being cordial; I did my best :-) –  J.R. Apr 17 '12 at 16:48
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