Hit the Sigma/Sum button, ∑, then enter the LaTeX markup of a math expression.

For example, try entering this:

c = \pm\sqrt{a^2 + b^2}

or this:

f(x) = \int_{-\infty}^\infty \hat f(\xi)\,e^{2 \pi i \xi x} \,d\xi

But make sure you select a good spot for it down here somewhere, as it will insert inline.