/sci/ - Science & Math

How do you get the minus (marked in red) down there?

For the record: [math]\Delta (\tilde{x}) = \vert x-\tilde{x} \vert[/math] and [math]\delta (\tilde{x}) = \frac{\Delta (\tilde{x})}{\vert \tilde{x} \vert}[/math].