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 :

$$(5)\sum_{i=1}^{n}{{B_4\left(x_i\right)y}}$$ $$x = {-b x_i \pm \sqrt{b^2-4ac} \over 2a}.$$

$$ g\left(x\right)=y_{i-\ }B_0\left(x_i\right)C_0-B_1\left(x_i\right)C_1-\ B_2\left(x_i\right)C_2-\ B_3\left(x_i\right)C_3-\ B_4\left(x_i\right)C_4 $$

This does not :

$$ (5)\sum_{i=1}^{n}{{B4\left(xi\right)y}{i\ }=\sum{i=1}^{n}{B_0{\left(x_i\right)B}_4\left(x_i\right)C0}+\sum{i=1}^{n}{B_1{\left(x_i\right)B}_4\left(x_i\right)C1}+\ \sum{i=1}^{n}{B_2{\left(x_i\right)B}_3\left(x_i\right)C2}+\ \sum{i=1}^{n}{B_3{\left(x_i\right)B}_4\left(x_i\right)C2}+\ \sum{i=1}^{n}{{B_4}^2\left(x_i\right)C_4}} $$

$$ \frac{\partial E}{\partial C4}=\sum{i=1}^{n}{\frac{\partial}{\partial C4}(y{i-\ }B_0\left(x_i\right)C_0-B_1\left(x_i\right)C_1-\ B_2\left(x_i\right)C_2-\ B_3\left(x_i\right)C_3-\ B_4\left(x_i\right)C_4)}^2=0$$

$$ \frac{\partial E}{\partial C0}=\sum{i=1}^{n}2\left(y_{i-\ }B_0\left(x_i\right)C_0-B_1\left(x_i\right)C_1-\ B_2\left(x_i\right)C_2-\ B_3\left(x_i\right)C_3-\ B_4\left(x_i\right)C_4\right).\left(-B_0\left(x_i\right)\right)=0 $$