# How to write matrix in equation environment?

I'm using LaTeX to write my paper and every equation needs to be numbered, so I use the equation environment. However, it fails to compile.

\documentclass[UTF8]{ctexart}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{epstopdf}
\begin{document}
$$$\begin{bmatrix} L_{a1} & 0 \\ 0 & L_{a2} \end{bmatrix} \begin{bmatrix} I_{a1} \\ I_{a2} \end{bmatrix} + \begin{bmatrix} R_{a1} & 0 \\ 0 & R_{a2} \end{bmatrix} \begin{bmatrix} I_{a1} \\ I_{a2} \end{bmatrix} + \begin{bmatrix} K_{e1} & 0 \\ 0 & K_{e2} \end{bmatrix} \begin{bmatrix} theta_1 \\ theta_2 \end{bmatrix} = \begin{bmatrix} V_{a1} \\ V_{a2} \end{bmatrix}$$$
\end{document}


So my question is: how to revise it? In addtion, I would like to know why the math expression that follows is wrong:

$$B = \frac{J_2}{N_1 N_2}+\frac{m_2 r_2^2}{4 N_1 N_2}+\frac{m_2 r_1 r_2}{2 N_1 N_2} cos(\frac{\theat_{m2}}{N_2})$$
$$C = \frac{-m_2 r_1 r_2}{N_1 N_2}(\frac{\theat_{m1} \theat_{m2}}{N_1}+\frac{\theat_{m2}^2}{2 N_2}) sin\frac{\theat_{m2}}{N_2}$$
$$D = J_{m1} +\frac{J_2}{N_2^2}+\frac{m_2 r_2^2}{4 N_2^2}$$
$$E = \frac{J_2}{N_1 N_2}+\frac{m_2 r_2^2}{4 N_1 N_2}+\frac{m_2 r_1 r_2}{2 N_1 N_2} cos(\frac{\theat_{m2}}{N_2})$$


LaTeX gives the information:

Undefined control sequence. [..._2}{2 N_1 N_2} cos(\frac{\theat_{m2}}{N_2}]

• \theta not \theat. Also you don't need  inside equation environment. – Sigur Jan 7 '14 at 15:44
• And for the last bit, don't use $$...$$. See e.g. Why is $…$ preferable to $$? – Torbjørn T. Jan 7 '14 at 15:47 • Also remove  from inside bmatrix (you are already in math mode). Also you probably want \theta and not theta – karlkoeller Jan 7 '14 at 15:48 • Since you're just starting to learn LaTeX I recommend that you write your math just a little bit at a time, compiling often. That way your mistakes will show up just one at a time and will be easier to find and fix. When you're stuck after trying, ask here, one question at a time. – Ethan Bolker Jan 7 '14 at 16:21 ## 1 Answer The following code should work. Changes: • deleted $ in line 8 • deleted $ in line 40 • deleted  in lines 32 and 33 -> you're already in a math environment, so you don't need to mark this as a math environment by using  ..  • added \ in front of theta in lines 32 and 33 \documentclass[UTF8]{ctexart} \usepackage{amsmath} \usepackage{amssymb} \usepackage{graphicx} \usepackage{epstopdf} \begin{document} $$\begin{bmatrix} L_{a1} & 0 \\ 0 & L_{a2} \end{bmatrix} \begin{bmatrix} I_{a1} \\ I_{a2} \end{bmatrix} + \begin{bmatrix} R_{a1} & 0 \\ 0 & R_{a2} \end{bmatrix} \begin{bmatrix} I_{a1} \\ I_{a2} \end{bmatrix} + \begin{bmatrix} K_{e1} & 0 \\ 0 & K_{e2} \end{bmatrix} \begin{bmatrix} \theta_1 \\ \theta_2 \end{bmatrix} = \begin{bmatrix} V_{a1} \\ V_{a2} \end{bmatrix}$$ \end{document}  Regarding your equations: You wrote theat instead of theta -> changing that should do it: $$ B = \frac{J_2}{N_1 N_2}+\frac{m_2 r_2^2}{4 N_1 N_2}+\frac{m_2 r_1 r_2}{2 N_1 N_2} cos(\frac{\theta_{m2}}{N_2})  C = \frac{-m_2 r_1 r_2}{N_1 N_2}(\frac{\theta_{m1} \theta_{m2}}{N_1}+\frac{\theta_{m2}^2}{2 N_2}) sin\frac{\theta_{m2}}{N_2}  D = J_{m1} +\frac{J_2}{N_2^2}+\frac{m_2 r_2^2}{4 N_2^2}  E = \frac{J_2}{N_1 N_2}+\frac{m_2 r_2^2}{4 N_1 N_2}+\frac{m_2 r_1 r_2}{2 N_1 N_2} cos(\frac{\theta_{m2}}{N_2})