# How to manage this long fraction in latex

$$\pi(\tau|x)= \frac{ B(S_{\tau}+a,b) \,_2F_1(S_{\tau}+\tau r, S_{\tau}+a, S_{\tau}+a+b; -\frac{1}{r})} {\sum_{\tau=1}^{n} B(S_{\tau}+a,b) \,_2F_1(S_{\tau}+\tau r, S_{\tau}+a, S_{\tau}+a+b; -\frac{1}{r})}$$

$$\times \frac{B(S_{n-\tau}+a_1,b_1) \,_2F_1(S_{n-\tau}+(n-\tau) r, S_{n-\tau}+a_1, S_{n-\tau}+a_1+b_1; -\frac{1}{r}) }{B(S_{n-\tau}+a_1,b_1) \,_2F_1(S_{n-\tau}+(n-\tau) r, S_{n-\tau}+a_1, S_{n-\tau}+a_1+b_1; -\frac{1}{r})}$$

• What is the question? – Jack Nov 14 '16 at 12:45
• Welcome to TeX.SX! Even if the title gives hint on what your asking, being a bit more explicit would help us help you! – ebosi Nov 14 '16 at 12:50

I assume, that you wish to merge both equation in one, two line equation. In this the use of package amsmath or mathtools (improved version of amsmath) and their math environments like multline, align etc are handy:

\documentclass{article}
\usepackage[margin=30mm]{geometry}
\usepackage{mathtools}

\usepackage{showframe}% for show page layout, in real use had to be omitted
\renewcommand*\ShowFrameColor{\color{red}}

\begin{document}
\begin{multline}
\pi(\tau|x) =
\frac{B(S_{\tau}+a,b)_2 F_1(S_{\tau}+\tau r, S_{\tau}+a, S_{\tau}+a+b; -\frac{1}{r})}
{\sum\limits_{\tau=1}^{n} B(S_{\tau}+a,b)_2
F_1(S_{\tau}+\tau r, S_{\tau}+a, S_{\tau}+a+b; -\frac{1}{r})}
\times\\
\frac{B(S_{n-\tau}+a_1,b_1)_2F_1(S_{n-\tau}+(n-\tau)r, S_{n-\tau}+a_1, S_{n-\tau}+a_1+b_1;  -\frac{1}{r}) }
{B(S_{n-\tau}+a_1,b_1)_2F_1(S_{n-\tau}+(n-\tau)r, S_{n-\tau}+a_1, S_{n-\tau}+a_1+b_1;  -\frac{1}{r})}
\end{multline}
or
$$\pi(\tau|x) = \frac{B(S_{\tau}+a,b)_2 F_1(S,\tau,r)} {\sum\limits_{\tau=1}^{n} B(S_{\tau}+a,b)_2 F_1(S,\tau,r)} \times \frac{B(S_{n-\tau}+a_1,b_1)_2F_1(S,n,\tau,r)} {B(S_{n-\tau}+a_1,b_1)_2F_1(S,n,\tau,r)}$$
where
\begin{align*}
F_1(S,\tau,r)   & = F_1(S_{\tau}+\tau r,      S_{\tau}+a,     S_{\tau}+a+b;       -\frac{1}{r})\\
F_1(S,n,\tau,r) & = F_1(S_{n-\tau}+(n-\tau)r, S_{n-\tau}+a_1, S_{n-\tau}+a_1+b_1; -\frac{1}{r})
\end{align*}
\end{document}


Let mi noted, that second part of equation is equal 1 :) (or you have some error in it).