fraction in latex with long items [duplicate]

$$\frac{\partial \mathcal{L}}{\partial \eta_1}= -\sum_{i=1}^{n}\frac{\frac{\eta_1}{c}-\cos(y_i-\beta_i X_i)}{c-\eta_1\cos(y_i-\beta_i X_i)-\eta_2\sin(y_i-\beta_i X_i)}$$


I need a big denominator and numerator.

You can use the \dfrac (displaystyle fraction) command from amsmath or \mfrac (medium-sized fraction, ca 80% of displaystyle) (from nccmath):

\documentclass{article}
\usepackage[utf8]{inputenc}}
\usepackage{mathtools, nccmath}

\newcommand*{\e}{\mathrm{e}}

\begin{document}

$\frac{\partial \mathcal{L}}{\partial \eta_1}= -\sum_{i=1}^{n}\frac{\dfrac{\eta_1}{c}-\cos(y_i-\beta_i X_i)}{c-\eta_1\cos(y_i-\beta_i X_i)-\eta_2\sin(y_i-\beta_i X_i)}$%

$\frac{\partial \mathcal{L}}{\partial \eta_1}= -\sum_{i=1}^{n}\frac{\mfrac{\eta_1}{c}-\cos(y_i-\beta_i X_i)}{c-\eta_1\cos(y_i-\beta_i X_i)-\eta_2\sin(y_i-\beta_i X_i)}$%

$\frac{\partial \mathcal{L}}{\partial \eta_1}= -\sum_{i=1}^{n}\frac{\frac{\eta_1}{c}-\cos(y_i-\beta_i X_i)}{c-\eta_1\cos(y_i-\beta_i X_i)-\eta_2\sin(y_i-\beta_i X_i)}$%

\end{document}