can be produced by:
-b±√[over]b[sup]2[/sup] -4ac[/over] [over] 2a [/over]
But that's slow to type and difficult to edit, with the bbCodes overwhelming the actual text, so I've defined short forms for several bbCodes as follows:
[over] = [o]
[strike] = [s]
[sup] = [p] (suPer or Power)
[sub] = [r] (Radix)
So the above can also be produced by:
-b±√[o]b[p]2[/p] -4ac[/o] [o] 2a [/o]
I note that what happens to the overline above a superscript, like the 2 above, depends on your browser.
The ± and √ symbols were typed, using WinCompose, as ⎄+- and ⎄v/
The following page was very helpful in creating this [over] bbCode. It also talks about MathJax, which allows the use of TeX.
If anyone can find a MathJax extension validated for phpBB version 3.3, please let me know.
[Edit: Changed [e] to [p] as per following posts.]