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

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