This is an example: $$x+1=\frac{x}{3}$$
\begin{equation*}
y=\frac{\sum_{i=0}^{n}(x_{i}^{2})}{x_{i}}\label{eq:3}
\end{equation*}
When \(a \ne 0\), there are two solutions to \(ax^2 + bx + c = 0\) and they are
\(x = {-b \pm \sqrt{b^2-4ac} \over 2a}.\)
This is Equation \ref{eq:1}
here is a lebeled equation:$$x+1\over\sqrt{1-x^2}\label{eq:1}$$
\begin{align*}
x&=y_1-y_2+y_3-y_5-\dots
&& \text{by \eqref{eq:1}}\\
&=y'\circ y^* && \text{(by \eqref{eq:1})}\\
& y{0}y' *\text{by Axiom1.}
\end{align*}
This is another equation\ref{ref3}
\begin{align}
a &=b\label{ref3}\\
y &=c+d
\end{align}