In mathematics, a usually represents an unknown or variable. When solving equations and solving mathematical problems, A can represent any numerical value, so A is also called a placeholder. For example, if we want to solve equation 2a+3=7, and A represents an unknown number, we need to find the value of A through mathematical operation, and finally get a=2.

There is another way to express a, that is, a stands for the first term in geometric series. Geometric series is a common mathematical model, in which the results of every term except the previous one are equal, and this equal ratio is called common ratio. The first term is the first term in geometric series, which is usually represented by a. By the first term A and the common ratio R, we can find any term in geometric series, for example, the n term is an = a * (r (n- 1)).

In addition to the above two meanings, a can also mean the average of $x_i$ in mathematics, that is, the arithmetic average. In a data set containing n elements, where each element consists of $x_ 1, x_2, ..., x_n$, the average value of this data set is a = ($ \ sum _ {i =1} n $ $ x _ i $)/n Arithmetic average is the most common statistic in mathematics.