$\displaystyle \chi^2 = \sum_{i=1}^N
\left(y_i - (a + b x_i)
\over \sigma_i\right)^2 $

tex TEX ${x^i}\over{\tan y}$ $a @\ b$ $@code{math code} a < b$ $\underline{@code{math \hbox{ code }}} \sum_{i}{\underline{f}}$ $\sum_{i} f(x)@sup{2}$

$\displaystyle -- f(x) = {1 \over \sigma \sqrt{2\pi}}e@sup{-{1 \over 2}\left({x-\mu \over \sigma}\right)^2}
$