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Answers for “两个集合 A 和 B 的笛卡尔积是什么?”

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Meet the concept

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The Cartesian product of two sets A and B, denoted A×BA \times B, is the set of all possible ordered pairs (x, y) where the first element x belongs to set A and the second element y belongs to set B.

Conditions: A and B are arbitrary sets.

Find a method

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The video tests the less-than relation with two reversed ordered pairs: (2, 5) and (5, 2). It shows that 2<52 < 5 is true, so the pair (2, 5) belongs to the relation.

Conditions: The relation is the less-than symbol (<) on integers.; The pairs tested are (2, 5) and (5, 2).

Meet the concept

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No, a relation does not have to contain every possible ordered pair. By definition, a relation is a subset of the Cartesian product A×BA \times B, meaning it can include only some of the ordered pairs.

Conditions: A and B are sets.; R is a relation between A and B.

Understand why

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Ordered pairs are used because the order of the elements matters in a relation. The first component of the pair is related to the second component, and swapping them can change whether the relationship holds.

Conditions: The relation is between two sets A and B.; The relationship is directional or asymmetric.

Meet the concept

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In the definition of a relation, capital letters A and B denote the sets themselves, while lowercase letters a and b denote individual elements belonging to those sets. The ordered pair (a, b) represents a specific element of the Cartesian product A×BA \times B, where a is an element of A and b is an element of B.

Conditions: The context is the formal definition of a relation R between sets A and B.; The notation (a, b) ∈ A×BA \times B is used.

Meet the concept

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Yes, elements of the sets can be left out of a relation. A relation is a subset of the Cartesian product, so it does not need to include every possible ordered pair.

Conditions: A relation R is defined between sets A and B.; R is a subset of A×BA \times B.

Meet the concept

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A relation R between two sets A and B is formally defined as a subset of the Cartesian product A×BA \times B. This means that R is a collection of ordered pairs, where the first element of each pair belongs to set A and the second element belongs to set B.

Conditions: A and B are sets.; R is a binary relation between A and B.

Find a method

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The distributive law (A ∪ B) × C=(A×C)C = (A \times C) ∪ (B×CB \times C) can be understood by treating sets as lengths of line segments and Cartesian products as areas of rectangles. The area of a large rectangle with width A ∪ B and height C equals the sum of the areas of two smaller rectangles with widths A and B, and height C.

Conditions: A, B, and C are arbitrary sets.; The geometric analogy treats sets as lengths and Cartesian products as rectangular areas.

Find a method

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The property (A×BA \times B) ∩ (C×DC \times D) = (A ∩ C) × (B ∩ D) can be understood by representing Cartesian products as rectangles. The intersection of two rectangles is a smaller rectangle whose width is the intersection of the widths of the original rectangles, and whose height is the intersection of their heights.

Conditions: A, B, C, and D are arbitrary sets.; The geometric analogy treats sets as lengths and Cartesian products as rectangular areas.

Meet the concept

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The Cartesian product distributes over both union and intersection from both the left and the right. Specifically, (A ∪ B) × C=(A×C)C = (A \times C) ∪ (B×CB \times C), (A ∩ B) × C=(A×C)C = (A \times C) ∩ (B×CB \times C), A × (B ∪ C) = (A×BA \times B) ∪ (A×CA \times C), and A × (B ∩ C) = (A×BA \times B) ∩ (A×CA \times C).

Conditions: A, B, and C are arbitrary sets.

Find a method

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To compute the Cartesian product of two finite sets, systematically pair each element of the first set with every element of the second set, and list all resulting ordered pairs in a new set.

Conditions: The sets are finite and explicitly given.

Understand why

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No, (A×BA \times B) ∪ (C×DC \times D) generally does not equal (A ∪ C) × (B ∪ D). While the intersection of two Cartesian products is a Cartesian product, their union is typically an irregular shape (like an L-shape) that cannot be expressed as a single Cartesian product of two sets.

Conditions: A, B, C, and D are arbitrary sets.