How do I write mathematical symbols and expressions in able2know ?

Well, if it's mathematical symbols you want to write, you can use the Alt key + numbers. You can find the whole list of combinations, just google "alt code list of alt key codes symbols". Now, this will write all the symbols in line, I don't know any way to make stuff like a fraction, or a proper square root. But the kind of symbols you can write are ± ∞ ≥ ² ∩ ≡ Σ among others.

I hope newer versions of this site will support some kind of mathematical formatting.

In the meantime, what I often do is google the name of the symbol, find it on Wikipedia, and cut and paste it into my post.