here \TeX

\ifx \HCode \undefined
\else
\HCode{\Hnewline <!-- tex4ht_begin tex_tex4ht_tex tex 1 -->}
\fi

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

\bye