Long equation with left and right justified

Question

Good day to all,

I have below equation, the first equation exceeds my margin if written in one line. So I want it appear in two lines which the first line is justified at the left side and the second line is justified at the right. Is this is a good practice or I have to put it in other way, which is accepted by mathematician especially.

\documentclass{report}
\usepackage{amssymb,amsmath}

\begin{document}

\begin{eqnarray}\nonumber
\mathbf{c}_m^t(b_0\mathbf{I}_m +b_{1}\mathbf{Q}_{m}+b_{2}\mathbf{Q}^2_{m}+\cdots +b_n \mathbf{Q}_m^n)\mathbf{H}_m=&\\ \label{panjang}
=\mathbf{eQ}_m^{\alpha}(a_0 \mathbf{I}_m^{}+a_1\mathbf{Q}_m+a_2\mathbf{Q}_m^{2}+\cdots+a_n\mathbf{Q}_m^{n})\mathbf{H}_m
\end{eqnarray}
where \(\mathbf{e}=\begin{bmatrix}\sqrt{m} & 0 & \cdots & 0\end{bmatrix}\). Rewrite Eqn. \ref{panjang} with
\begin{eqnarray}g_1(\mathbf{Q}_{m})=a_0 \mathbf{I}_m+a_1\mathbf{Q}_m^{}+a_2\mathbf{Q}_m^{2}+\cdots+a_n\mathbf{Q}_m^{ n}\end{eqnarray} 
and 
\begin{eqnarray}
g_2(\mathbf{Q}_{m})=b_0\mathbf{I}_m +b_{1}\mathbf{Q}_{m}+b_{2}\mathbf{Q}^2_{m}+\cdots +b_n \mathbf{Q}_m^n\end{eqnarray} it becomes
\begin{eqnarray}
\mathbf{c}_m^t g_{2}(\mathbf{Q}_{m})&=&
\mathbf{e}\mathbf{Q}_m^\alpha g_1(\mathbf{Q}_{m}).
\end{eqnarray}

\end{document}

Answer 1

As mentioned for example in the UK TeX FAQ the use of eqnarray is discouraged.

To achieve the layout you want, you can use for example the multline environment from amsmath. What else can be done is described for example in the Short Math Guide for LaTeX. I would not put a = on the end of the first line. But otherwise I think your approach is the best way to go in this case.

Answer 2

I would certainly replace the first eqnarray environment with a multline environment, as @canaaerus suggests in his answer.

Furthermore, I would also replace the second and third eqnarray environments with a single align environment (with the word and inserted with a \shortintertext macro), and I'd replace the final eqnarray environment with a simple equation environment. Basically, there's no good reason for using eqnarray, especially as the available alternatives are so much better.

In addition, I'd create a macro to abbreviate the very frequently occurring \mathbf{Q} with something shorter, say, \Q, i.e., I set up the macro \newcommand{\Q}{\mathbf{Q}}. Finally, if you want to have parentheses placed around cross-referenced equation numbers, you can get LaTeX to do this for you automatically if you use the \eqref (instead of \ref).

Taken together, the following modified MWE results:

\documentclass{report}
\usepackage{mathtools}
\numberwithin{equation}{chapter} % just for the MWE
\newcommand{\Q}{\mathbf{Q}}
\begin{document}
\setcounter{chapter}{5} % just for the MWE
\setcounter{equation}{19} % just for the MWE
\begin{multline}\label{panjang}
\mathbf{c}_m^t(b_0\mathbf{I}_m +b_{1}\Q_{m}+b_{2}\Q^2_{m}+\dots +b_n \Q_m^n)\mathbf{H}_m\\ 
=\mathbf{eQ}_m^{\alpha}(a_0 \mathbf{I}_m^{}+a_1\Q_m+a_2\Q_m^{2}+\dots+a_n\Q_m^{n})\mathbf{H}_m
\end{multline}
where \(\mathbf{e}=
\begin{bmatrix}\sqrt{m} & 0 & \dots & 0\end{bmatrix}\). Rewrite Eqn.~\eqref{panjang} with
\begin{align}g_1(\Q_{m})&=a_0 \mathbf{I}_m+a_1\Q_m^{}+a_2\Q_m^{2}+\dots+a_n\Q_m^{ n}
\shortintertext{and}
g_2(\Q_{m})&=b_0\mathbf{I}_m +b_{1}\Q_{m}+b_{2}\Q^2_{m}+\dots +b_n \Q_m^n
\end{align} 
so that it becomes
\begin{equation}
\mathbf{c}_m^t g_{2}(\Q_{m}) =
\mathbf{e}\Q_m^\alpha g_1(\Q_{m}).
\end{equation}
\end{document}