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

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

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