Chapter 1 - Section 1.2 - Algebraic Expressions and Sets of Numbers - Exercise Set - Page 16: 40

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The set {1, 2, 3, ...} defines all natural numbers, or "counting" numbers. The number 12 is included in that set. Think about continuing the set and you will find the number 12: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ...}