# Big matrices: how to write RHS in a new line

I am trying to write a system of the form Ax=By, where A and B are "big matrices". Here my code:

\begin{equation*}
\begin{flushleft}
\begin{bmatrix}
\textbf{I} & -\beta\,\Delta t\,\textbf{I} & \textbf{0}\\[0.35cm]
\beta\,\Delta t\,\textbf{E} & \textbf{M} & -\beta\,\Delta t\,\textbf{B} \\[0.35cm]
\textbf{0} & \beta\,\Delta t\,\textbf{B}^\mathrm{T} &\overline{\textbf{C}}+\beta \,\Delta t\,\overline{\textbf{K}}\,
\end{bmatrix}
\begin{bmatrix}
\{u^{(n+1)}\}\\[0.35cm]
\{v^{(n+1)}\} \\[0.35cm]
\{\vartheta^{(n+1)}\}
\end{bmatrix}
=\\[0.2cm]
\end{flushleft}
\begin{flushright}
=\begin{bmatrix}
\textbf{I}& (1-\beta)\,\Delta t\,\textbf{I} & \textbf{0}\\[0.35cm]
-(1-\beta)\,\Delta t\,\textbf{E} & \textbf{M} & (1-\beta)\,\Delta t\,\textbf{B} \\[0.35cm]
\textbf{0}& -(1-\beta) \,\Delta t\,\textbf{B}^\mathrm{T} &\overline{\textbf{C}}- (1-\beta)\,\Delta t\,\overline{\textbf{K}}\,
\end{bmatrix}
\begin{bmatrix}
\{u^{(n)}\}\\[0.35cm]
\{v^{(n)}\} \\[0.35cm]
\{\vartheta^{(n)}\}
\end{bmatrix},
\end{flushright}
\end{equation*}


I am getting some error (e.g. Bad math environment delimiter. \end{equation*}). How to fix it? Any suggestion for a better formatting? Thanks!

Some suggestions:

• Use a multline* environment with a single \\ line-break directive.
• Get rid of the flushleft and flushright environments, especially as they are meant to be used in text mode, not in math mode.
• Replace all \textbf instances with \mathbf.
• Reset \arraystretch to a value of 1.5 -- and get rid of all [0.35cm] spacing directives
• Optional: Get rid of \, thinspace directives

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

\begin{multline*}
\renewcommand\arraystretch{1.5} % default value: 1.0
\begin{bmatrix}
\mathbf{I} & -\beta\Delta t\mathbf{I} & \mathbf{0}\\
\beta\Delta t\mathbf{E} & \mathbf{M} & -\beta\Delta t\mathbf{B} \\
\mathbf{0} & \beta\Delta t\mathbf{B}^\mathrm{T} &\overline{\mathbf{C}}+\beta \Delta t\overline{\mathbf{K}}
\end{bmatrix}
\begin{bmatrix}
\{u^{(n+1)}\}\\
\{v^{(n+1)}\} \\
\{\vartheta^{(n+1)}\}
\end{bmatrix}
\\[1ex]
=\begin{bmatrix}
\mathbf{I}& (1-\beta)\Delta t\mathbf{I} & \mathbf{0}\\
-(1-\beta)\Delta t\mathbf{E} & \mathbf{M} & (1-\beta)\Delta t\mathbf{B} \\
\mathbf{0}& -(1-\beta) \Delta t\mathbf{B}^\mathrm{T} &\overline{\mathbf{C}}- (1-\beta)\Delta t\overline{\mathbf{K}}
\end{bmatrix}
\begin{bmatrix}
\{u^{(n)}\}\\
\{v^{(n)}\} \\
\{\vartheta^{(n)}\}
\end{bmatrix},
\end{multline*}

\end{document}

• Thank you! I also added \renewcommand\arraystretch{1.5} in between \[1ex] and =\begin{bmatrix}.
– Mik
Commented Feb 3, 2018 at 8:22

The simplest for this one, in my opinion, is to use the multline* environment:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[showframe]{geometry}
\usepackage{amsmath}

\begin{document}

\begin{multline*}
\begin{bmatrix}
\textbf{I} & -\beta\,\Delta t\,\textbf{I} & \textbf{0}\\[0.35cm]
\beta\,\Delta t\,\textbf{E} & \textbf{M} & -\beta\,\Delta t\,\textbf{B} \\[0.35cm]
\textbf{0} & \beta\,\Delta t\,\textbf{B}^\mathrm{T} &\overline{\textbf{C}}+\beta \,\Delta t\,\overline{\textbf{K}}\,
\end{bmatrix}
\begin{bmatrix}
\{u^{(n+1)}\}\\[0.35cm]
\{v^{(n+1)}\} \\[0.35cm]
\{\vartheta^{(n+1)}\}
\end{bmatrix}
=\\[1ex]
=\begin{bmatrix}
\textbf{I}& (1-\beta)\,\Delta t\,\textbf{I} & \textbf{0}\\[0.35cm]
-(1-\beta)\,\Delta t\,\textbf{E} & \textbf{M} & (1-\beta)\,\Delta t\,\textbf{B} \\[0.35cm]
\textbf{0}& -(1-\beta) \,\Delta t\,\textbf{B}^\mathrm{T} &\overline{\textbf{C}}- (1-\beta)\,\Delta t\,\overline{\textbf{K}}\,
\end{bmatrix}
\begin{bmatrix}
\{u^{(n)}\}\\[0.35cm]
\{v^{(n)}\} \\[0.35cm]
\{\vartheta^{(n)}\}
\end{bmatrix},
\end{multline*}

\end{document}


This is a minimal damage kit to your equation.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\begin{align*}
\MoveEqLeft
\begin{bmatrix}
\textbf{I} & -\beta\,\Delta t\,\textbf{I} & \textbf{0}\\[0.35cm]
\beta\,\Delta t\,\textbf{E} & \textbf{M} & -\beta\,\Delta t\,\textbf{B} \\[0.35cm]
\textbf{0} & \beta\,\Delta t\,\textbf{B}^\mathrm{T} &\overline{\textbf{C}}+\beta \,\Delta t\,\overline{\textbf{K}}\,
\end{bmatrix}
\begin{bmatrix}
\{u^{(n+1)}\}\\[0.35cm]
\{v^{(n+1)}\} \\[0.35cm]
\{\vartheta^{(n+1)}\}
\end{bmatrix}
=\\[0.2cm]
=&\begin{bmatrix}
\textbf{I}& (1-\beta)\,\Delta t\,\textbf{I} & \textbf{0}\\[0.35cm]
-(1-\beta)\,\Delta t\,\textbf{E} & \textbf{M} & (1-\beta)\,\Delta t\,\textbf{B} \\[0.35cm]
\textbf{0}& -(1-\beta) \,\Delta t\,\textbf{B}^\mathrm{T} &\overline{\textbf{C}}- (1-\beta)\,\Delta t\,\overline{\textbf{K}}\,
\end{bmatrix}
\begin{bmatrix}
\{u^{(n)}\}\\[0.35cm]
\{v^{(n)}\} \\[0.35cm]
\{\vartheta^{(n)}\}
\end{bmatrix},
\end{align*}
\end{document}