Only replying to this because there have been lots of views and someone posted recently, showing continuing interest.
The new pages version has the ability to to do exactly this on iOS. It has a Paragraph sign above the normal keyboard that has a new option to add an equation and that new keyboard layout has the +/- on it .
that functionality does not seem to be present in any other of the apps though in ios11
Not sure why nobody suggested this up to now, but there is also at least one free keyboard you can download that does have the +/- sign you are referring to on it. (I am not associated with it, note it does have an in app purchase option for more characters i think). those additional keyboards are accessible from any program I believe.
Jason