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If we agree that $\textbf{(a) }\dfrac{x}{x}=1$, $\textbf{(b) }\dfrac{0}{x}=0$, and that $\textbf{(c) }\dfrac{x}{0}=\infty^{\large\dagger}$, and let us suppose $z=0$:

$$\begin{align*} z&=0&&\text{given.}\\ \dfrac{z}{z}&=\dfrac{0}{z}&&\text{divide each side by }z.\\ \dfrac{z}{z}&=0&&\text{by }\textbf{(b)}.\\ 1&=0&&\text{by }\textbf{(a)}. \end{align*}$$ Now, from this, we can get say that $\dfrac{0}{0}=1$. What went wrong?


$\dagger$ Or undefined, if you prefer it.

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    $\begingroup$ If you assume that the inverse of $0$ exists in a ring (with unity), then it's standard proving that $0$ is the only element. $\endgroup$
    – egreg
    Dec 17, 2014 at 22:34
  • $\begingroup$ (a) is only true if $x\neq 0$ because other you get your proof to show $0=1$ $\endgroup$
    – Squirtle
    Dec 17, 2014 at 22:35
  • $\begingroup$ @egreg Are you referring to Ring Theory? :/ $\endgroup$
    – Conor O'Brien
    Dec 17, 2014 at 22:47
  • $\begingroup$ @ConorO'Brien Yes; but since the integers form a ring where $0\ne1$, the inverse of $0$ doesn't exist, unless you prefer to violate the distributive law. $\endgroup$
    – egreg
    Dec 17, 2014 at 22:48
  • $\begingroup$ @egreg erm... could you explain to the non-Ring theorist what exactly that means? $\endgroup$
    – Conor O'Brien
    Dec 17, 2014 at 22:48

6 Answers 6

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All answers were given, I'll give one answer offering additional information on division by zero.

Actually it's possible to divide by zero. It's usually not possible to divide by zero, because this operation is meaningless(in some contexts). But sometimes, it's useful to be able to divide by zero and there is a theory devoted to it, called Wheel Theory. Have a look at this article too. Just be careful on how do you use this information, there are contexts in which this is useful. At some of them, it's useless.

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  • $\begingroup$ Hmm... very interesting. Thanks! $\endgroup$
    – Conor O'Brien
    Dec 17, 2014 at 23:01
  • $\begingroup$ Very interesting indeed. $\endgroup$
    – Miguelgondu
    Dec 17, 2014 at 23:07
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You cannot divide by zero. So if $z=0$, then you also cannot divide by $z$. Therefore, your second equality $$\frac{z}{z}=\frac0z$$ is ultimately incorrect.

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Thou shalt not divide by zero. Your first statement doesn't hold if $x=0$.

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  • $\begingroup$ It's actually possible to divide by zero. Take a look at my answer. $\endgroup$
    – Red Banana
    Dec 17, 2014 at 23:09
  • $\begingroup$ Very interesting! I'd never heard of such beasts. $\endgroup$
    – John
    Dec 17, 2014 at 23:18
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First, there are a couple of comments to make.

(a) is only valid for $x \neq 0$; (b) is only vaild for $x \neq 0$ (c) is not defined.

So you don't have the right to divide by $z$ in your second line.

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We agree about a little more than what you wrote. We agree that

(a) If $x\neq 0$, then $\frac xx=1$

(b) If $x\neq 0$, then $\frac0x = 0$

We do not agree about $(c)$ at all, we think it is undefined.

Using only (a) and (b), you cannot prove that $\frac00=1.$

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You divided by zero on the second line. There are many proofs similar to yours that end up being complete garbage because of division by zero. Although your example doesn't hide it as well.

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  • $\begingroup$ It's actually possible to divide by zero. Take a look at my answer. $\endgroup$
    – Red Banana
    Dec 17, 2014 at 23:10
  • $\begingroup$ Interesting, +1. $\endgroup$
    – MathMajor
    Dec 17, 2014 at 23:11

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