Here's one using the equation environment: $$\begin{equation} x+1\over\sqrt{1-x^2} \end{equation}$$ and one with equation* environment: $$\begin{equation*} x+1\over\sqrt{1-x^2} \end{equation*}$$

This is a forward reference [$\ref{ref2}$] and another $\eqref{ref2}$ for the following equation: $$x+1\over\sqrt{1-x^2}\label{ref2}$$ More math: $$x+1\over\sqrt{1-x^2}$$ Here is a ref inside math: $\ref{ref2}+1$ and text after it. $$\begin{align} x& = y_1-y_2+y_3-y_5+y_8-\dots && \text{by \eqref{ref1}}\\ & = y'\circ y^* && \text{(by \eqref{ref3})}\\ & = y(0) y' && \text {by Axiom 1.} \end{align}$$ Here's a bad ref [$\ref{ref4}$] to a nonexistent label.

An alignment: $$\begin{align} a&=b\label{ref3}\cr &=c+d \end{align}$$ and a starred one: $$\begin{align*} a&=b\cr &=c+d \end{align*}$$