Here is a labeled equation \eqref{eq1}: $$x+1\over\sqrt{1-x^2}\label{eq1}$$ and another equation: $$\frac{1}{3}\cdot\sum_{i=1}^{n} x^3\nonumber$$ Eq test: \begin{equation} f(x)=\frac{ax^2+bx+c}{e^{x+2}} \end{equation} Alignment test: \begin{align*} x&= y_1-y_2+y_3-y_5+y_8-\dots \\ &= y(0) y'\; \text{by Axiom 1.} \nonumber\\ &= \text{by Eq.\eqref{eq1}}\nonumber \\ &= y'\times y^\frac{n}{2} \\ \end{align*}