Writing theorems and definitions

LaTeX provides environment for theorems, definitions, and alike. Returning to our first example in this chapter, we could define a theorem environment by:

\newtheorem{thm}{Theorem}

Then, we declare a definition environment, using an optional argument stating an existing environment with which we would like to share the numbering:

\newtheorem{dfn}[thm]{Definition}

And in this simple way, we may use these environments:

\begin{dfn}
A quadratic equation is an equation of the form
\begin{equation}
  \label{quad}
ax^2 + bx + c = 0,
\end{equation}
where \( a, b \) and \( c \) are constants and \( a \neq 0 \).
\end{dfn}
\begin{thm}
A quadratic equation (\ref{quad}) has two solutions for the variable \( x \):
\begin{equation}
  \label{root}
  x_{1,2} = \frac{-b \pm \sqrt{b^2-4ac}}{2a}. 
\end{equation}
\end{thm}

In the output, such environments are numbered and labeled "Theorem" and "Definition", respectively. In Chapter 10, Developing Large Documents, when we are going...