\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[a4paper]{geometry} % change later to \usepackage{geometry}
\geometry{verbose,tmargin=1cm,bmargin=1cm,lmargin=1cm,rmargin=1cm,
headheight=0cm,headsep=0cm,footskip=0cm,nomarginpar}
\pagestyle{empty}
\setlength{\parskip}{0bp}
\setlength{\parindent}{0pt}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{setspace}
\usepackage{babel}
\usepackage{ulem}
\begin{document}
\def\arraystretch{2}
\[ \begin{array}{rcl}
C_\theta H^T [HC_\theta H^T + C_\eta]^{-1} &=& [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1} \\
C_\theta H^T [HC_\theta H^T + C_\eta]^{-1}[HC_\theta H^T + C_\eta] &=& [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1}[HC_\theta H^T + C_\eta] \\
C_\theta H^T &=& [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1}[HC_\theta H^T + C_\eta] \\
[C_\theta^{-1} + H^T C_\eta^{-1} H] C_\theta H^T & = & [C_\theta^{-1} + H^T C_\eta^{-1} H][C_\theta^{-1} + H^T C_\eta^{-1} H]^{-1} H^T C_\eta^{-1} [H C_\theta H^T + C_\eta] \\
\end{array} \]
\end{document}
给出的错误信息是：
! Missing number, treated as zero.
<to be read again>
C
l.269 [C_\theta^{-1} + H^T C_\eta^{-1} H]
C_\theta H^T & = & [C_\theta^{-1} ...
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)
! Illegal unit of measure (pt inserted).
<to be read again>
C
l.269 [C_\theta^{-1} + H^T C_\eta^{-1} H]
C_\theta H^T & = & [C_\theta^{-1} ...
Dimensions can be in units of em, ex, in, pt, pc,
cm, mm, dd, cc, nd, nc, bp, or sp; but yours is a new one!
I'll assume that you meant to say pt, for printer's points.
To recover gracefully from this error, it's best to
delete the erroneous units; e.g., type `2' to delete
two letters. (See Chapter 27 of The TeXbook.)
! Missing = inserted for \ifdim.
<to be read again>
C
l.269 [C_\theta^{-1} + H^T C_\eta^{-1} H]
C_\theta H^T & = & [C_\theta^{-1} ...
I was expecting to see `<', `=', or `>'. Didn't.
! Missing number, treated as zero.
<to be read again>
C
l.269 [C_\theta^{-1} + H^T C_\eta^{-1} H]
C_\theta H^T & = & [C_\theta^{-1} ...
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)
! Illegal unit of measure (pt inserted).
<to be read again>
C
l.269 [C_\theta^{-1} + H^T C_\eta^{-1} H]
C_\theta H^T & = & [C_\theta^{-1} ...
Dimensions can be in units of em, ex, in, pt, pc,
cm, mm, dd, cc, nd, nc, bp, or sp; but yours is a new one!
I'll assume that you meant to say pt, for printer's points.
To recover gracefully from this error, it's best to
delete the erroneous units; e.g., type `2' to delete
two letters. (See Chapter 27 of The TeXbook.)
当我删除下面的粗体部分（第 269 行）时，它可以正常工作：
\[ \begin{array}{rcl}
C_\theta H^T [HC_\theta H^T + C_\eta]^{-1} &=& [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1} \\
C_\theta H^T [HC_\theta H^T + C_\eta]^{-1}[HC_\theta H^T + C_\eta] &=& [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1}[HC_\theta H^T + C_\eta] \\
C_\theta H^T &=& [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1}[HC_\theta H^T + C_\eta] \\
**[C_\theta^{-1} + H^T C_\eta^{-1} H]** C_\theta H^T & = & [C_\theta^{-1} + H^T C_\eta^{-1} H][C_\theta^{-1} + H^T C_\eta^{-1} H]^{-1} H^T C_\eta^{-1} [H C_\theta H^T + C_\eta] \\
当我在不同位置运行相关部分时，它没有出现任何错误：
\[ % No errors given for this:
[C_\theta^{-1} + H^T C_\eta^{-1} H] C_\theta H^T = [C_\theta^{-1} + H^T C_\eta^{-1} H][C_\theta^{-1} + H^T C_\eta^{-1} H]^{-1} H^T C_\eta^{-1} [H C_\theta H^T + C_\eta] \]
\end{array} \]
我的代码有什么问题？
答案1
错误之处相当微妙：\\查找[包含间距选项的以下内容。因此，\\ \relax在最后一行之前可以解决问题。
但排版方程式的更好方法是使用align*：
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
C_\theta H^T [HC_\theta H^T + C_\eta]^{-1} &= [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1} \\
C_\theta H^T [HC_\theta H^T + C_\eta]^{-1}[HC_\theta H^T + C_\eta] &=[C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1}[HC_\theta H^T + C_\eta] \\
C_\theta H^T &= [C_\theta^{-1}+H^TC_\eta^{-1}H]^{-1}H^TC_\eta^{-1}[HC_\theta H^T + C_\eta] \\
[C_\theta^{-1} + H^T C_\eta^{-1} H] C_\theta H^T & = [C_\theta^{-1} + H^T C_\eta^{-1} H][C_\theta^{-1} + H^T C_\eta^{-1} H]^{-1} H^T C_\eta^{-1} [H C_\theta H^T + C_\eta] \\
\end{align*}
\end{document}
答案2
在数组中，LaTeX 会在 之后查找可选参数\\，因此如果接下来是方括号，则会出错。尝试\null 在数组最后一行的开头插入。