Prevent automatic resizing of equation elements [duplicate]

I have the following equation:

$$E(W) = \frac{\frac{(c\rho)^c}{c!}}{(1-\rho)\sum_{n=0}^{c-1}(\frac{(c\rho)^n}{n!})+\frac{(c\rho)^c}{c!}} \cdot \frac{1}{1-\rho} \cdot \frac{1}{c\mu}$$


which is rendered as

Is there any easy way to prevent that the nominator and denominator of the main fraction to be each displayed in inline math mode? E.g. I want my summation to look like this instead of the way it looks right now.

• You can use \displaymath for example. Nov 30, 2020 at 21:11
• but as far as I understand this the equation is already in display math, or is there a way to force display math for the individual elements of the equation aswell? Nov 30, 2020 at 21:12
• add \limits directly after the \sum command \sum\limits_{n=0}^{c-1} ... Nov 30, 2020 at 21:15
• Sebastiano meant \frac{\displaymath ...}{\displaymath ...} so numerator and denominator are set in display style Nov 30, 2020 at 21:16
• Thanks all of you this has helped me! Nov 30, 2020 at 21:24

Here there is my version using also \displaystyle and \dfrac. I not very good for the explanations :-).
\documentclass[a4paper,12pt]{article}
$$E(W) = \frac{\dfrac{(c\rho)^c}{c!}}{(1-\rho)\displaystyle\sum_{n=0}^{c-1}\left(\frac{(c\rho)^n}{n!}\right)+\frac{(c\rho)^c}{c!}} \cdot \frac{1}{1-\rho} \cdot \frac{1}{c\mu}$$