# Arithmetic question regarding $\sqrt{1/-1} = \sqrt{-1/1}$ [duplicate]

Can someone please point out what I'm doing wrong here?

$$\frac{1}{-1} = \frac{-1}{1} \implies \sqrt{\frac{1}{-1}} = \sqrt{\frac{-1}{1}}$$ which should imply that $$\frac{\sqrt{1}}{\sqrt{-1}} = \frac{\sqrt{-1}}{\sqrt{1}}$$

But the last equality is not true. Is there something really obvious that I'm overlooking here?

## marked as duplicate by J. M. is a poor mathematician, GEdgar, user2468, Ross Millikan, M TurgeonAug 22 '12 at 17:13

• The issue is that $\sqrt{ab}\neq \sqrt{a}\sqrt{b}$. This question has been asked here before in numerous variations. – Alex Becker Aug 22 '12 at 16:42
• $\sqrt{ab} = \sqrt{a}\sqrt{b}$ only holds when $a, b \ge 0.$ Hence $\sqrt{\frac{1}{-1}} \neq \frac{\sqrt{1}}{\sqrt{-1}}.$ – user2468 Aug 22 '12 at 16:56
Assume that $f \colon A \to B$ is a given function. From $x=y$ you can get $f(x)=f(y)$, but only when $x$ and $y$ belong to $A$. In your example, $f=\sqrt{\cdot}$, $A=[0,+\infty)$ and $B=\mathbb{R}$. But $x=-1/1 = -1 \notin A$, and you can't conclude.
Of course, you may say that $A =\mathbb{C}$, but then $f$ becomes multi-valued.