How do you input maths into a section header?

Question

so I am currently in the process of making a summary for my maths subject. I'm a new TeX user and using pdfTeX on TeXworks.

I want to make this to show x=f(y) in the header as it would look like with maths typesetting in the body. What code will I need to use so that x=f(y) looks like it's maths, rather than normal paragraph text as a section header?

My code I already have is below.

\documentclass{article}
\usepackage{geometry}
\usepackage{fancyhdr}
\usepackage{amsmath,amsthm,amssymb}
\usepackage{graphicx}
\usepackage{hyperref}
\usepackage{lipsum}
\usepackage{ifpdf}

And after other stuff

\section{Summary of Rules and Formulas}

\section{Differentiation}

\subsection{Preliminary Knowledge}
\label{labelone}

\subsection{{\[x=f(y)\]}}
\label{labeltwo}