Mathematical formulae are very poor in services such as Windows Word, Keynote, Pages -- or many times totally missing, not to speak about compability -problems. So how can I create and add formulae to such services in iPad?
I answer here about this case, to keep this question here more focused.
I have probably tested now all apps and -- the only apps I can recommend are MathBot and TeX Equation. Unfortunately, the TeX Equation has a small bug as can be seen in the examples. Below you can see how poorer products such as TeXit work. The worst case is with Equations -app, the one above TeXit in the photo.
MathJax VS TeX Equation