In LaTeX, you can use _{} for subscripts and ^{} for superscripts.

Examples: 注意前后要加$

1. **Single Subscript**:
$ x_{i} $  ，

2. **Single Superscript**:
$ x^{2} $     

3. **Subscript and Superscript Together**:
$ x_{i}^{2} $     

4. **Complex Expressions**:
$ x_{i+1} $ and $ x^{n+1} $   

Combined Example:

Here is a more complex expression with both subscripts and superscripts:

$ a_{n} = b^{n} + c_{i} $   

Fraction Example:

1. **Simple Fraction**:
$ \frac{a}{b} $   

2. **Fraction with Expression**:
$ \frac{n(n-1)}{2} $   

3. **Fraction with Powers**:
$ \frac{x^2}{y^3} $ 

4. **Fraction with Subscripts**:
$ \frac{a_{n}}{b_{n+1}} $

5. **Fraction with Complex Numerator and Denominator**:
$ \frac{x_{i+1} + y}{z_{j} + w} $