Probably easiest syntax for MathJax is AsciiMath (but it also supports Latex, MathML etc.).

AsciiMath is written inside forward-ticks, like this:

´sum_(i=1)^n i^3=((n(n+1))/2)^2´

Result is: ´sum_(i=1)^n i^3=((n(n+1))/2)^2´