1. Letters can represent any number, but only one number can be represented in a question.
2. It can also represent a formula with a specific meaning, a certain number that meets the conditions, or even a number with certain rules. In short, letters can express the quantitative relationship concisely.
3. For example, A can represent a set; F (x) is a function of x, etc.
4. Letters representing numbers is a great development in the history of mathematics.
