# 高中数学——映射与变换

## 映射

$f(x)=\vert x \vert,x \in A$

## 序偶

(a,b)中的$a$，$b$不仅仅表示一个数字，就抽象出了序偶的概念。

## 笛卡尔积

$$\begin{cases} \forall a \in A, \exists b \in B \Rightarrow {(a,b)} \in C \\ {(a,b_1)} \in C, {(a,b_2)} \in C \Rightarrow b_1=b_2 \end{cases}$$
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