Dividing an equation into two lines

Question

I am trying to divide this equation into two different lines but I am not able to. I tried to use package breqn but that started giving me some compilation errors on the style file itself.

\frac{ \sum_{n_i\in S_{c_k}} P_{ij} ( x_{i} P(y_i = 1\ |\ x_i = 1) + (1- x_{i}) P (y_i = 1\ |\ x_i=0))}{\sum_{n_i\in S_{c_k}} P_{ij}}

The problem here is \frac{} which prevents me from using align to break the equation.

Answer 1

Since you mention that this is a simplified version of your actual formula, I am assuming that it won't fit on one line. If so, here is one way you can display a similar equation over multiple lines:

Code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
\frac{1}{\sum_{n_i\in S_{c_k}} P_{ij}}
\Biggl[
\sum_{n_i\in S_{c_k}} &\phantom{+}P_{ij} \bigl( x_{i} P(y_i = 1\ |\  x_i = 1 ) \\
    &+ (1- x_{i}) P(y_i = 1\ |\ x_i=0)\bigr) \Biggr]
\end{align*}
\end{document}

Answer 2

You can use multline

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{multline}
a+b+c+d+e+f\\
+i+j+k+l+m+n
\end{multline}

\end{document}