\mml 1

\begin {equation}f_X(x) = \lambda e^{-\lambda x} u(x). \label {eqlam}\end {equation}

37C135BEA9A3826439E1346D4F81E60D

$$\begin{array}{cc}\text{(1.1)}& {𝑓}_{𝑋}(𝑥)=𝜆{𝑒}^{-𝜆𝑥}𝑢(𝑥).\end{array}$$

\mml 2

\begin {equation}a^2 + b^2 = c^2.\end {equation}

140A9001CF6C1467DD9F64540FF17EC0

$$\begin{array}{cc}\text{(A.1)}& {𝑎}^2+{𝑏}^2={𝑐}^2.\end{array}$$

\mml 3

\begin {equation}a^2 + b^2 = c^2.\end {equation}

140A9001CF6C1467DD9F64540FF17EC0

$$\begin{array}{cc}\text{(A.2)}& {𝑎}^2+{𝑏}^2={𝑐}^2.\end{array}$$