Split equation problem

Question

I would like to split the following equation in two parts. Very basic thing, seems to be.

\begin{equation}
\zeta_k(x) = \frac{\sin (\frac{1}{2} (x - x_0)) \cdots \sin(\frac{1}{2} (x-x_{k-1}))}
{ \sin ( \frac{1}{2} (x_k - x_0)) \cdots \sin(\frac{1}{2} (x_k - x_{k-1}))} %
%
\frac{\sin ( \frac{1}{2} (x - x_{k+1})) \cdots %
\sin(\frac{1}{2} (x - x_{2n}))}{\sin(\frac{1}{2} (x_k - x_{k+1})) \cdots %
\sin(\frac{1}{2} (x_k - x_{2n}))}
\label{eq3}
\end{equation}

I inserted amsmath package and tried with begin split and so on but can not get to the point where I separate the equation into two lines, one that contains

\zeta_k(x) = \frac{\sin (\frac{1}{2} (x - x_0)) \cdots \sin(\frac{1}{2} (x-x_{k-1}))}
{ \sin ( \frac{1}{2} (x_k - x_0)) \cdots \sin(\frac{1}{2} (x_k - x_{k-1}))} %
%

and the other that contains

\frac{\sin ( \frac{1}{2} (x - x_{k+1})) \cdots %
\sin(\frac{1}{2} (x - x_{2n}))}{\sin(\frac{1}{2} (x_k - x_{k+1})) \cdots %
\sin(\frac{1}{2} (x_k - x_{2n}))}

any suggestions?

Answer 1

I'd use (memoir not relevant)

\documentclass[a4paper]{memoir}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{split}
\zeta_k(x) ={} & \frac{\sin (\frac{1}{2} (x - x_0)) \cdots
  \sin(\frac{1}{2} (x-x_{k-1}))} { \sin ( \frac{1}{2} (x_k - x_0))
  \cdots \sin(\frac{1}{2} (x_k - x_{k-1}))} %
 %
\\
& \times \frac{\sin ( \frac{1}{2} (x - x_{k+1})) \cdots %
  \sin(\frac{1}{2} (x - x_{2n}))}{\sin(\frac{1}{2} (x_k -
  x_{k+1})) \cdots %
  \sin(\frac{1}{2} (x_k - x_{2n}))}
\end{split}
\label{eq3}
\end{equation}
\end{document}