我怎样才能将段落固定在相同的缩进内？

Question

\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}
\relpenalty=9999
\binoppenalty=9999
\renewcommand{\qedsymbol}{$\blacksquare$}
\newcommand*{\QEDA}{\hfill\ensuremath{\blacksquare}}%


% SYMBOLS %
\def\loc{{\textstyle{\rm loc}}}
\def\C{{\bf C}}                            % complex numbers
\def\R{{\mathbb R}}                            % real numbers
\def\Rhat{{\widehat{\R}}}                  % reals (dual)
\def\N{{\mathbb N}}                            % natural numbers
\def\Z{{\mathbb Z}}                            % integers
\def\Q{{\bf Q}}

%%%%%%%%%%%  MACROS  %%%%%%%%%%%%

% UNARY, BINARY OPERATORS %
\def\norm#1{\|  #1 \|}

\def\CHI{\hbox{\raise .5ex \hbox{$\chi$}}}
\def\bmu{{\mu}}
\def\bnu{{\nu}}
\def\n{{\bf n}}
\def\x{{\bf x}}
\def\y{{\bf y}}
\def\p{{\bf p}}
\def\r{{\bf r}}
\def\X{{\cal X}}
\def\Y{{\cal Y}}
\def\T{{\cal T}}
\def\S{{\cal S}}

\begin{document}

\noindent \textbf{2)} Let $f:\R^2 \to \R$ by the formula 
\[
  f(\x) =
  \begin{cases}
    \frac{x_1^2x_2}{x_1^4+x_2^2},  & \text{ if } \x \neq \mathbf{0} \\
    0,    & \text{ if } \x = \mathbf{0} 
  \end{cases}
\]

\textbf{a)} Show that if $\x \to \mathbf{0}$ along either the $x_1$- or the $x_2$-coordinate axis, $f(\x)\to 0$.

\end{document}

我想将 $f(\mathbf{x}) \to 0$ 的部分修复到 \textbf{a)} 之后第一个句子的相同缩进内，即“显示...”的正下方

Answer 1

这很容易enumitem。我还修复了代码中的一些缺陷。

例如，\bf已经被弃用了 20 多年。此外，使用\def非常危险，因为您可能会覆盖重要的命令。最好使用更语义化的命名，例如\bx“boldface x”。

\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{bm}
\usepackage{enumitem}

\usepackage{lipsum} % just for the example

%\relpenalty=9999
%\binoppenalty=9999
\renewcommand{\qedsymbol}{$\blacksquare$}
%\newcommand*{\QEDA}{\hfill\ensuremath{\blacksquare}}%


% SYMBOLS %
\newcommand\loc{\mathrm{loc}}
\newcommand\C{\mathbf{C}}        % complex numbers
\newcommand\R{\mathbb{R}}        % real numbers
\newcommand\Rhat{\widehat{\R}}   % reals (dual)
\newcommand\N{\mathbb{N}}        % natural numbers
\newcommand\Z{\mathbb{Z}}        % integers
\newcommand\Q{\mathbf{Q}}

%%%%%%%%%%%  MACROS  %%%%%%%%%%%%

% UNARY, BINARY OPERATORS %
\newcommand\norm[1]{\|#1\|}

\newcommand\CHI{\text{\raisebox{.5ex}{$\chi$}}}
\newcommand\bmu{\bm{\mu}}
\newcommand\bnu{\bm{\nu}}
\newcommand\bn{\mathbf{n}}
\newcommand\bx{\mathbf{x}}
\newcommand\by{\mathbf{y}}
\newcommand\bp{\mathbf{p}}
\newcommand\br{\mathbf{r}}
\newcommand\cX{\mathcal{X}}
\newcommand\cY{\mathcal{Y}}
\newcommand\cT{\mathcal{T}}
\newcommand\cS{\mathcal{S}}

%% Environments
\newenvironment{exercises}
 {\begin{enumerate}[label=\bfseries\arabic*),leftmargin=*,align=left]}
 {\end{enumerate}}
\newenvironment{questions}
 {\begin{enumerate}[label=\bfseries\alph*),leftmargin=*,align=left]}
 {\end{enumerate}}

\begin{document}

\begin{proof}
\lipsum*[2]
\end{proof}

\begin{exercises}
\item Let $f\colon\R^2 \to \R$ be defined by the formula 
\[
  f(\bx) =
  \begin{cases}
    \dfrac{x_1^2x_2}{x_1^4+x_2^2},  & \text{if $\bx \neq \mathbf{0}$} \\[2ex]
    0,    & \text{if $\bx = \mathbf{0}$} 
  \end{cases}
\]
  \begin{questions}
  \item Show that if $\bx \to \mathbf{0}$ along either 
        the $x_1$- or the $x_2$-coordinate axis, $f(\bx)\to 0$.
  \end{questions}
\end{exercises}

\end{document}

我添加了一个proof环境来展示如何\QEDA不需要。

Answer 2

添加到序言中\usepackage{enumerate}，并将该部分内容写如下：

\begin{enumerate}[{\bfseries 1)}]
    \item Let $f:\R^2 \to \R$ be defined by the formula 
    \[
      f(\x) =
      \begin{cases}
        \frac{x_1^2x_2}{x_1^4+x_2^2},  & \text{ if } \x \neq \mathbf{0} \\
        0,    & \text{ if } \x = \mathbf{0} 
      \end{cases}
    \]
    \begin{enumerate}[{\bfseries a)}]
        \item Show that if $\x \to \mathbf{0}$ along either the $x_1$- or the $x_2$-coordinate axis, $f(\x) \to 0$.
        \item Show that also ..
    \end{enumerate}
\end{enumerate}

如果您确实想将其推$f(\mathbf{x}) \to 0$至新行（我不推荐），只需\\在其前面添加即可。