Formula 1: $$ \begin{equation} \label{Formula1.Label} P (\Omega) = \int_{-\infty}^{+\infty} f (x) \: dx = 1 \end{equation} $$

Formula 2: $$ \begin{equation} \label{Formula2.Label} E (X) := \int_{-\infty}^{+\infty} x \: f (x) \; dx \end{equation} $$

A third one, with references to 1 and 2: $$ \begin{equation} \begin{array}{ll} E (a X + b) & = \int_{-\infty}^{+\infty} (a x - b) \: f(x) \: dx \\ \\ & = a \int_{-\infty}^{+\infty} x \: f(x) \: dx + b \int_{-\infty}^{+\infty} f(x) \: dx \\ \\ & \underset{\eqref{Formula1.Label}, \eqref{Formula2.Label}}{=} a E(X) + b \end{array} \end{equation} $$

And another one: $$ \begin{equation} V(X) = \frac{a^2 + b^2 + c^2 - ab - ac - bc}{18} \end{equation} $$