Yes. Tex is on. I found that selection in the administration section. That's why some equations were rendering. The problem was other (identically formatted) equations where not rendering. It was very mysterious.
I hunted a little more and read a comment and answer that suggested 'mimetex.linux' needed 755 permissions. I changed that and now equations are rendering. There is an annoying (but not fatal) feature now:
If I leave a blank space between the dollar signs delimiters, those render as [?]. This can be remedied by not leaving any blank space, on the mark-up side but I find that makes things more challenging to proof read on the mark-up side. White space is useful!
If someone has other tips for making Tex more 'friendly', that would be great. (My guess is many of the 'tips' involve: "run 2.8", but for now that's not an option!)
Still, if there are some tips on things to put in a Tex preamble? If yes... or info on where/how that is done in moodle? That would be great!
Sorry to be such a newbie, but I'd like the equations pretty.
That said: I can live with the current behavior.
On Mathjax:The Mathjax page for 2.5 shows 'blank', so I assume it's not available. https://docs.moodle.org/25/en/MathJax_filter