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Question:

I am writing a long equation inside align which produced two labels ((1) and (2)) actually this is only one equation. How can I label this only one label at center?

MWE:

\documentclass[12pt]{article}
\usepackage[a4paper,top=0.8 in,bottom=0.8 in,left=0.6 in,right=0.5 in]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[misc]{ifsym}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{delarray}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc,shapes}
\usepackage{parskip}
\usepackage{cellspace}
\setlength\cellspacetoplimit{4pt}
\setlength\cellspacebottomlimit{4pt}
\begin{document}
\large
\begin{align}
u\;\dfrac{\partial u}{\partial x} + v\;\dfrac{\partial v}{\partial y} &=\left(\dfrac{\mu + K}{\rho}\right)\;\dfrac{\partial^2 u}{\partial y^2} + \left(\dfrac{K}{\rho}\right)\;\dfrac{\partial N}{\partial y} + g\,\beta_{T}\,(T-T_{\infty})+g\,\beta_{C}\,(C-C_{\infty})\\[14pt]
&\hspace{6cm}-\dfrac{\sigma\,B_{0}^{2}(x)}{\rho}\;u-\dfrac{\mu}{\rho \;k'}\,u
\end{align}
\end{document}
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Two equation numbers -- one per row -- appear in the align environment because that's how the environment is designed.

If you want a single equation number that's placed halfway between the rows, I suggest you use either an equation/split or an equation/aligned combination. (The latter combination provides some extra capabilities, but they're not needed for your use case.)

For much more information about the multi-row display-math environments of the amsmath package, I strongly recommend you read section 3, "Displayed equations", of the package's user guide.

Separately, I would also like to recommend that you (a) omit the large, and unneeded, parentheses around some of the \frac terms and (b) replace \hspace{6cm} with \quad.

enter image description here

\documentclass[12pt]{article}
\usepackage[a4paper, vmargin=0.8in, left=0.6in, right=0.5in]{geometry}
%\usepackage[utf8]{inputenc} % that's been the default for several years by now
\usepackage[misc]{ifsym}

\usepackage{amsmath}
%\usepackage{amsfonts} % is loaded automatically by 'amssymb'
\usepackage{amssymb}

\usepackage{graphicx}
\usepackage{delarray}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{calc,shapes}

\usepackage{parskip}

\usepackage{cellspace}
\setlength\cellspacetoplimit{4pt}
\setlength\cellspacebottomlimit{4pt}

\begin{document}
\large
\begin{equation}
\begin{split}
u\,\frac{\partial u}{\partial x} + v\,\frac{\partial v}{\partial y} 
&=  \frac{\mu + K}{\rho}\,\frac{\partial^2 u}{\partial y^2} 
  + \frac{K}{\rho}\,\frac{\partial N}{\partial y} 
  + g\,\beta_{T}(T-T_{\infty})
  + g\,\beta_{C}(C-C_{\infty}) \\[1ex]
&\quad -\frac{\sigma B_{0}^{2}(x)}{\rho}\,u-\frac{\mu}{\rho k'}\,u
\end{split}
\end{equation}
\end{document}
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  • 1
    @ Mico This is outstanding.......Thanks for your valuable suggestions and reference. – Math-Man Jan 19 at 6:42

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