Mathjax notation not always working

It's possible to use Mathjax syntax to embed math equations in a post. This works well for simple ones, but fails for a lot of the ones I want to use. This works : $$(5)\sum_{i=1}^{n}{{B_4\left(x_i\right)y}}$$ $$x = {-b x_i \pm \sqrt{b^2-4ac} \over 2...