Problem splitting/fitting equation

I am struggling with an equation that exceeds the page, as it is very long. I have already read that the best solution is splitting the equation, so I have tried using {multline} or {split}, but I keep getting errors. Can anybody help me?

    \begin{equation}
\begin{aligned}
i\to -\frac{-c \left(\beta  (-2 \gamma -\delta +\nu )+\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta  c (\nu +\mu ) (2 \gamma +\delta +\mu )}\right)+\beta  \left(\beta  \delta +\beta  \mu +\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta  c (\nu +\mu ) (2 \gamma +\delta +\mu )}\right)+c^2 (\nu +\mu )}{2 \beta ^2 c}
\end{aligned}
\end{equation} Thank you :-)

I would reorganize the terms and avoid the big fraction:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{split}
i\to \frac{1}{2 \beta^2 c}
\Bigl(
& c\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2
+2 \beta  c (\nu +\mu ) (2 \gamma +\delta +\mu )}
\\
& -\beta\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2
+2 \beta  c (\nu +\mu ) (2 \gamma +\delta +\mu )}
\\
& +c\beta (-2 \gamma -\delta +\nu )
-\beta^2 (\delta + \mu)
-\beta c^2 (\nu +\mu )
\Bigr)
\end{split}
\end{equation}

\end{document} Here are two possibilities (+ a variant), loading mathtools for themultlinedenvironment and thesplitfraccommand, andnccmath for its medium-size commands, combining them in various ways:

\documentclass{article}
\usepackage[showframe]{geometry}
\usepackage{mathtools, nccmath}

\begin{document} %

\begin{equation}
\begin{multlined}[b]i\to -\frac1{2 \beta ^2 c}\biggl[(-c \Bigl(\beta (-2 \gamma -\delta +\nu )+\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta c (\nu +\mu ) (2 \gamma +\delta +\mu )}\Bigr) \\+\beta \Bigl(\beta \delta +\beta \mu +\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta c (\nu +\mu ) (2 \gamma +\delta +\mu )}\Bigr)+c^2 (\nu +\mu )\biggr]
\end{multlined}
\end{equation}
\bigskip
\begin{equation}
\begin{aligned}
i\to -\frac{\medmath{\splitfrac{-c \left(\beta (-2 \gamma -\delta +\nu )+\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta c (\nu +\mu ) (2 \gamma +\delta +\mu )}\right) + }{\beta \left(\beta \delta +\beta \mu +\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta c (\nu +\mu ) (2 \gamma +\delta +\mu )}\right)+c^2 (\nu +\mu )}}}{2 \beta ^2 c}
\end{aligned}
\end{equation}
\bigskip
\begin{equation}
\begin{aligned}
i\to \medmath{-\frac{\splitfrac{-c \left(\beta (-2 \gamma -\delta +\nu )+\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta c (\nu +\mu ) (2 \gamma +\delta +\mu )}\right) + }{\beta \left(\beta \delta +\beta \mu +\sqrt{\beta ^2 (\delta +\mu )^2+c^2 (\nu +\mu )^2+2 \beta c (\nu +\mu ) (2 \gamma +\delta +\mu )}\right)+c^2 (\nu +\mu )}}{2 \beta ^2 c}}
\end{aligned}
\end{equation}

\end{document}
` • Thank you so much! This really helped me! With your comment I was able to solve my problem! – Isi Apr 23 '18 at 21:02