Hi, ngureco, I noticed that jamesroy11143 answered your question nicely, but if you want a lower-tech way just so you can get the formula in your hub quickly, here is what I did:
1. If you don't already have the formula in picture format, generate the formula some other way (e.g Microsoft Word) and take a screen shot of just that part of your screen.
2. Upload that picture of the formula as a photo capsule in your hub. OK so it's low-tech but it works!
If you want to see what a formula entered that way can look like, here's see my hub here: http://hubpages.com/hub/Calculate-Keyword-Phrase-D...
(look about halfway down, in a paragraph entitled "A better manual formula for phrases"). Am truly not trying to self-promote here, my 2-step method relates specifically to hubs containing formulas and it does help if you can see what the result can look like in a published hub containing a formula that used that method!