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*}
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*}