Intersection Distributes over Union/Venn Diagram
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Theorem
Set intersection is distributive over set union:
- $R \cap \paren {S \cup T} = \paren {R \cap S} \cup \paren {R \cap T}$
Proof
Demonstration by Venn diagram:
In the left hand diagram, $R$ is depicted in yellow and $S \cup T$ is depicted in blue.
Their intersection $R \cap \paren {S \cup T}$ where they overlap is depicted in green.
In the right hand diagram, $\paren {R \cap S}$ is depicted in yellow and $\paren {R \cap T}$ is depicted in blue.
Their intersection, where they overlap, is depicted in green.
Their union is the total shaded area: yellow, blue and green.
As can be seen by inspection, the areas are the same.
Sources
- 1965: Seth Warner: Modern Algebra ... (previous) ... (next): Chapter $\text I$: Algebraic Structures: $\S 3$: Unions and Intersections of Sets
- 2008: David Joyner: Adventures in Group Theory (2nd ed.) ... (previous) ... (next): Chapter $1$: Elementary, my dear Watson: $\S 1.2$: Elements, my dear Watson: Ponderable $1.2.1 \ \text{(a)}$