Here is a labeled equation:
\[\begin{equation} x+1\over\sqrt{1-x^2} \label{ref1} \end{equation}\]with a reference to ref1: \(\ref{ref1}\) , and another numbered one with no label: \[x+1\over\sqrt{1-x^2}\]
This one uses \nonumber: \[x+1\over\sqrt{1-x^2}\nonumber\]
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:
\[\begin{equation} x+1\over\sqrt{1-x^2}\label{ref2} \end{equation}\]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:
\[\begin{align*} a&=b\cr &=c+d \end{align*}\]