Fraction not working for a long equation

Question

I am trying to write this equation

So far, I have this:

 \begin{equation}\label{EEDIeq}
\resizebox{0.85\hsize}{!}{\frac{{\Bigg(\prod_{j=1}^n f_j\Bigg) 
    \Bigg(\sum_{i=1}^{nME}P_{ME(i)}.C_{FME(i)}.SFC_{ME(i)}\Bigg) + 
        (P_{AE}.C_{FAE}.SFC_{AE})+ 
    \Bigg(\Bigg(\prod_{j=1}^n f_j.\sum_{i=1}^{nPTI} P_{PTI(i)}-
    \sum_{i=1}^{neff}f_{eff(i)}.P_{AEeff(i)}\Bigg)
    C_{FAE}.SFC_{AE}\Bigg) -
    \Bigg(\sum_{i=1}^{neff}f_{eff(i)}.P_{eff(i)}.C_{FME}.SFC_{ME}\Bigg)}}{f_i.f_c.f_l.Capacity.f_w.V_{ref}}}
\end{equation}

But I can't get the denominator where I want it to be :

How can I fix this? Thanks :)

Answer 1

You shouldn't force your readers to look for a magnifying glass; it's better to split the thing across lines.

\documentclass{article}
\usepackage{amsmath,mathtools}
\newcommand{\vr}[1]{\mathit{#1}}

\begin{document}

\begin{equation}\label{EEDIeq}
\begin{split}
&\frac{1}{f_i\cdot f_c\cdot f_l\cdot \vr{Capacity}\cdot f_w\cdot V_{\vr{ref}}} \\
& \cdot\biggl[
   \biggl(\,\prod_{j=1}^n f_j\biggr)
   \biggl(\,\sum_{i=1}^{n_{\vr{ME}}} P_{\vr{ME}(i)}\cdot
     C_{\vr{FME}(i)}\cdot \vr{SFC}_{\vr{ME}(i)}\biggr)\\
&\qquad +
   (P_{\vr{AE}}\cdot C_{\vr{FAE}}\cdot \vr{SFC}_{\vr{AE}}) \\
&\qquad +
   \biggl(\biggl(\,\prod_{j=1}^n f_j\cdot \sum_{i=1}^{n_{\vr{PTI}}} P_{\vr{PTI}(i)}-
     \sum_{i=1}^{n_{\vr{eff}}} f_{\vr{eff}(i)}\cdot P_{\vr{AEeff}(i)}\biggr)
     C_{\vr{FAE}}\cdot \vr{SFC}_{\vr{AE}}\biggr) \\
&\qquad -
    \biggl(\,\sum_{i=1}^{n_{\vr{eff}}} f_{\vr{eff}(i)}\cdot P_{\vr{eff}(i)}\cdot
    C_{\vr{FME}}\cdot \vr{SFC}_{\vr{ME}}\biggr)
\biggr]
\end{split}
\end{equation}

\end{document}

The formatting of superscripts should be studied in greater detail. Never use a period to denote multiplication.

Answer 2

Rather than try to reduce the size of the equation in order to make it fit on a single line, I would employ a multline environment and typeset the equation across four lines: Three lines for the numerator, and the fourth and final line for the denominator. Note that in the following solution, I've dispensed with \frac notation.

\documentclass{article}
\usepackage{amsmath} % for 'multline' env.
\begin{document}
\begin{multline}\label{EEDIeq}
\Biggl\{\biggl(\,\prod_{j=1}^n f_j\biggr) 
\biggl(\,\sum_{i=1}^{n_{\mathrm{\mathrm{ME}}}}
P_{\mathrm{\mathrm{ME}}(i)}\cdot 
C_{\mathrm{FME}(i)}\cdot 
\mathit{SFC}_{\mathrm{\mathrm{ME}}(i)}\biggr) 
+ P_{\mathrm{AE}}\cdot C_{\mathrm{FAE}}
\cdot \mathit{SFC}_{\!\mathrm{AE}}\\
+ \biggl[\biggl(\,\prod_{j=1}^n f_j\cdot 
\sum_{i=1}^{n_{\mathrm{PTI}}} P_{\mathrm{PTI}(i)}
-\sum_{i=1}^{n_{\mathrm{eff}}}f_{\mathrm{eff}(i)}\cdot 
P_{\mathrm{AE}_{\mathrm{eff}}(i)}\biggr)
C_{\mathrm{FAE}}\cdot \mathit{SFC}_{\!\mathrm{AE}}\biggr] \\
-\biggl(\,\sum_{i=1}^{n_{\mathrm{eff}}}f_{\mathrm{eff}(i)}
\cdot P_{\mathrm{eff}(i)}\cdot C_{\mathrm{FME}}
\cdot \mathit{SFC}_{\mathrm{ME}}\biggr) \Biggr\}\\
\Big/\Bigl\{f_i\cdot f_c\cdot f_l\cdot \mathit{Capacity}
\cdot f_w\cdot V_{\mathrm{ref}}\Bigr\}
\end{multline}
\end{document}