TITLE: Regarding usage of $\pm$ sign
QUESTION [6 upvotes]: I want to say that $x=1$ or $x=-1$, but we don't know which one (or we could possibly calculate it, but it is complicated). Anyhow, for my future proof all I need to know is that $x$ is either $1$ or $-1$. 
Is it OK to write it as $x=\pm 1$ ?
I am asking because it seems that meaning here is different from for example when we use $\pm$ in the formula of roots of quadratic  equations.  
If I can't use $\pm$, then is there some other way I can denote it?

REPLY [5 votes]: In identities like $\tan\left(\dfrac\pi4\pm\dfrac\theta2\right)=\sec\theta\pm\tan\theta$, the notation means that if the first is "$+$" then so is the second and if the first is "$-$" then so is the second.
In things like $\cos(\alpha\pm\beta) = \cos\alpha\cos\beta \mp \sin\alpha \sin\beta$, if the first is "$+$" then the second is "$-$" and vice-versa.
If you say that a poll shows that $40\%\pm2\%$ of voters will vote "yes" in next month's referendum, it might mean that those are the bounds of a $90\%$, or maybe $95\%$ confidence interval.  I've been known to write things like
$$
\text{The bounds of a $90\%$ confidence interval are } 122\pm6.97.
$$
If one were to write something like $x\in\{\pm1\}$, it might in some contexts reasonably be construed as meaning $x\in\{1,-1\}$.