\documentclass[12pt,letterpaper,oneside]{article}
\usepackage[top=1.0in, bottom=1.0in, left=1.0in, right=1.0in]{geometry}
\usepackage{fancyhdr}
\usepackage{mathtools}
\fancyhead{}
\fancyfoot{}
\fancyhead[R]{\thepage}
\renewcommand{\headrulewidth}{0.4pt}
\renewcommand{\footrulewidth}{0.4pt}


\begin{document}
\title{It's a Document!}
\author{}
\date{}
\maketitle
\clearpage
\section{Introduction to Some Document}
\fancyhead[L]{Introduction to Some Document}
\pagestyle{fancy}
It's text...nothing fancy here. Keep moving.
\begin{eqnarray}
A &=& \pi{}r^2\\
V &=& \frac{4}{3}\pi{}r^3
\end{eqnarray}
Woah. It's math!
\clearpage
\pagestyle{fancy}
Some more text.
\end{document}