A test of Equation References

Here is a labeled equation: $$x+1\over\sqrt{1-x^2}\label{ref1}$$ 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*}


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*}