%\documentclass{amsart}
\documentclass[12pt]{article}

\begin{document}

This is a sample document with an error in the equation
below.  Later, we show you how to reference an equation.

$$
f'(x)=\lim_{h\rightarrow 0}\left( \frac{f(x+h)-f(x)}{h} \right
$$


We also show 
how to reference an equation.  Here is an equation
that we will reference later.

%Use begin and end instead of $$.  Also, be sure
% to put in a label for the equation (your choice,
% but no spaces!)
\begin{equation}
\int_a^b f(x)\,dx = F(b)-F(a)
\label{FTC}
\end{equation}

Now later on when I want to say ``Look back at
that equation I typed earlier'', we would write:

Look back at Equation \ref{FTC}.


\end{document}
