# Split equation problem

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

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


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?

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What error messages are you seeing? When I put \begin{split} ... \\ ... \end{split} in the right places then I get a decent output. Can you post the exact code that you tried that didn't work? –  Andrew Stacey Mar 31 '11 at 11:12
(For "the right places", see daleif's answer.) –  Andrew Stacey Mar 31 '11 at 11:15
Just a minor suggestion: to improve readability I would use \sin\bigl(\frac{1}{2} (x - x_0)\bigr) instead of \sin(\frac{1}{2} (x - x_0)) (similar remark applies to all the other factors). –  Gonzalo Medina Mar 31 '11 at 17:29
I realized that the problem has to do with my mac tex installation, since compilation in linux finally worked –  flow Apr 1 '11 at 8:22
@gnu_drug amsmath is very stable; I don't think it could be MacTeX that was the problem... –  Will Robertson Apr 1 '11 at 13:00
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\documentclass[a4paper]{memoir}
$$\begin{split} \zeta_k(x) ={} & \frac{\sin (\frac{1}{2} (x - x_0)) \cdots \sin(\frac{1}{2} (x-x_{k-1}))} { \sin ( \frac{1}{2} (x_k - x_0)) \cdots \sin(\frac{1}{2} (x_k - x_{k-1}))} % % \\ & \times \frac{\sin ( \frac{1}{2} (x - x_{k+1})) \cdots % \sin(\frac{1}{2} (x - x_{2n}))}{\sin(\frac{1}{2} (x_k - x_{k+1})) \cdots % \sin(\frac{1}{2} (x_k - x_{2n}))} \end{split} \label{eq3}$$