How to number two equations as one

Question

\begin{align}
E\left(y_{i}^{T}|u_{1i}\succ-x_{1i}'\beta_{1},u_{2i}\succ-x_{2i}'\beta_{2}\right)&=\frac{\sigma}{\Psi\left(x_{1i}'\beta_{1},\tfrac{x_{2i}'\beta_{2}}{\sigma},\rho\right)}\left[\phi\left(\tfrac{x_{2i}'\beta_{2}}{\sigma}\right)\Phi\left(\tfrac{x_{1i}'\beta_{1}-\rho\tfrac{x_{2i}'\beta_{2}}{\sigma}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right.\\
&\qquad + \left. \rho\phi(x_{1i}'\beta_{1})\Phi\left(\tfrac{\tfrac{x_{2i}'\beta_{2}}{\sigma}-\rho x_{1i}'\beta_{1}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right]
\end{align}

The code above gives two equation numbers but I want only one since they belong to only one equation. How do I do that?

Answer 1

Option 1

\documentclass[preview,border=12pt,12pt]{standalone}% change it to your own document class
\usepackage[a4paper,margin=2cm,landscape]{geometry}% edit it to meet your preference
\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax% don't use this line in your production
\begin{equation}
\begin{split}
E\left(y_{i}^{T}|u_{1i}\succ-x_{1i}'\beta_{1},u_{2i}\succ-x_{2i}'\beta_{2}\right)
&=\frac{\sigma}{\Psi\left(x_{1i}'\beta_{1},\tfrac{x_{2i}'\beta_{2}}{\sigma},\rho\right)}\left[\phi\left(\tfrac{x_{2i}'\beta_{2}}{\sigma}\right)\Phi\left(\tfrac{x_{1i}'\beta_{1}-\rho\tfrac{x_{2i}'\beta_{2}}{\sigma}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right.\\
&\qquad + \left. \rho\phi(x_{1i}'\beta_{1})\Phi\left(\tfrac{\tfrac{x_{2i}'\beta_{2}}{\sigma}-\rho x_{1i}'\beta_{1}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right]
\end{split}
\end{equation}
\end{document}

Suggestion

To make the code more readable (to make you easier to locate what you want to modify), remove the unnecessary {} (such as replacing X_{a}^{b} with X_a^b) and align the equation code with a better pattern as follows.

\documentclass[preview,border=12pt,12pt]{standalone}% change it to your own document class
\usepackage[a5paper,margin=2cm,landscape]{geometry}% edit it to meet your preference
\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax% don't use this line in your production
\begin{equation}
\begin{split}
E\left(y_i^T|u_{1i}\succ-x_{1i}'\beta_1,u_{2i}\succ-x_{2i}'\beta_2\right)
&=
\frac{\sigma}{\Psi\left(x_{1i}'\beta_1,\tfrac{x_{2i}'\beta_2}{\sigma},\rho\right)}
\left[
    \phi\left(\tfrac{x_{2i}'\beta_2}{\sigma}\right)
    \Phi\left(\tfrac{x_{1i}'\beta_1-\rho\tfrac{x_{2i}'\beta_2}{\sigma}}{\left(1-\rho^2\right)^{1/2}}\right)
\right.\\
&
\qquad +
\left.
    \rho\phi(x_{1i}'\beta_1)\Phi\left(\tfrac{\tfrac{x_{2i}'\beta_2}{\sigma}-\rho x_{1i}'\beta_1}{\left(1-\rho^2\right)^{1/2}}\right)
\right]
\end{split}
\end{equation}
\end{document}

Answer 2

Option 2

Use multline instead of align (option 1 already in DonutE.Knot's answer...)

MWE

\documentclass{article}

\usepackage{amsmath}
\usepackage{hyperref}

\begin{document}

\begin{multline}
E\left(y_{i}^{T}|u_{1i}\succ-x_{1i}'\beta_{1},u_{2i}\succ-x_{2i}'\beta_{2}\right)=\frac{\sigma}{\Psi\left(x_{1i}'\beta_{1},\tfrac{x_{2i}'\beta_{2}}{\sigma},\rho\right)}\left[\phi\left(\tfrac{x_{2i}'\beta_{2}}{\sigma}\right)\Phi\left(\tfrac{x_{1i}'\beta_{1}-\rho\tfrac{x_{2i}'\beta_{2}}{\sigma}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right.\\
\qquad + \left. \rho\phi(x_{1i}'\beta_{1})\Phi\left(\tfrac{\tfrac{x_{2i}'\beta_{2}}{\sigma}-\rho x_{1i}'\beta_{1}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right]
\end{multline}

\end{document} 

---

EDIT

As Barbara Beeton states in her comment, the equation doesn't fit in the page, so it would be better to add another break, for example

\documentclass{article}

\usepackage{amsmath}
\usepackage{hyperref}

\begin{document}

\begin{multline}
E\left(y_{i}^{T}|u_{1i}\succ-x_{1i}'\beta_{1},u_{2i}\succ-x_{2i}'\beta_{2}\right)\\
=\frac{\sigma}{\Psi\left(x_{1i}'\beta_{1},\tfrac{x_{2i}'\beta_{2}}{\sigma},\rho\right)}\left[\phi\left(\tfrac{x_{2i}'\beta_{2}}{\sigma}\right)\Phi\left(\tfrac{x_{1i}'\beta_{1}-\rho\tfrac{x_{2i}'\beta_{2}}{\sigma}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right.\\
+ \left. \rho\phi(x_{1i}'\beta_{1})\Phi\left(\tfrac{\tfrac{x_{2i}'\beta_{2}}{\sigma}-\rho x_{1i}'\beta_{1}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right]
\end{multline}

\end{document}  

Answer 3

What about a simple use of \nonumber after the part of the equation which should not have a number (before the \\)

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{align}
    E\left(y_{i}^{T}|u_{1i}\succ-x_{1i}'\beta_{1},u_{2i}\succ-x_{2i}'\beta_{2}\right)&=\frac{\sigma}{\Psi\left(x_{1i}'\beta_{1},\tfrac{x_{2i}'\beta_{2}}{\sigma},\rho\right)}\left[\phi\left(\tfrac{x_{2i}'\beta_{2}}{\sigma}\right)\Phi\left(\tfrac{x_{1i}'\beta_{1}-\rho\tfrac{x_{2i}'\beta_{2}}{\sigma}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right.\nonumber\\
    &\qquad + \left. \rho\phi(x_{1i}'\beta_{1})\Phi\left(\tfrac{\tfrac{x_{2i}'\beta_{2}}{\sigma}-\rho x_{1i}'\beta_{1}}{\left(1-\rho^{2}\right)^{1/2}}\right)\right]
\end{align}
\end{document}