If Q(x) is the statement x + 1 > 2x, and the domain is the set of positive integers, which of the following best characterizes the following two statements : A) there exists x such that Q(x) is true B) for every x, -Q(x) is true Only A is true Only B is true Both A and B are true Both A and B are false

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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### Logic and Set Theory: Analyzing Statements

**Problem Statement:**
Given that \( Q(x) \) is the statement \( x + 1 > 2x \), and the domain is the set of positive integers, which of the following best characterizes the two statements below?

A) There exists \( x \) such that \( Q(x) \) is true.

B) For every \( x \), \( \neg Q(x) \) is true.

**Options:**
1. Only A is true.
2. Only B is true.
3. Both A and B are true.
4. Both A and B are false.

**Analysis:**

Firstly, we analyze the given statement \( Q(x) \):
\[
Q(x): x + 1 > 2x
\]

Simplifying \( Q(x) \):
\[
x + 1 > 2x \implies 1 > x
\]

Since \( x \) is a positive integer (from the domain definition), \( x \) must be greater than or equal to 1. Therefore, the inequality \( 1 > x \) is false for all positive integers \( x \).

**Investigating Statement A:**
Statement A posits that there exists an \( x \) such that \( Q(x) \) is true:
\[
\exists x (x + 1 > 2x)
\]

Because \( Q(x) \) simplifies to \( 1 > x \), and there is no positive integer \( x \) for which this is true, Statement A is false.

**Investigating Statement B:**
Statement B posits that for every \( x \), the negation of \( Q(x) \) is true:
\[
\forall x (\neg Q(x))
\]

Since \( Q(x) \) is equivalent to \( 1 > x \), its negation is:
\[
\neg Q(x):  x \geq 1
\]

This inequality \( x \geq 1 \) is true for all positive integers \( x \), hence Statement B is true.

**Conclusion:**
From the analysis, we can conclude that "Only B is true."

Thus, the correct option is:
\[
\boxed{\text{Only B is true.}}
\]
Transcribed Image Text:### Logic and Set Theory: Analyzing Statements **Problem Statement:** Given that \( Q(x) \) is the statement \( x + 1 > 2x \), and the domain is the set of positive integers, which of the following best characterizes the two statements below? A) There exists \( x \) such that \( Q(x) \) is true. B) For every \( x \), \( \neg Q(x) \) is true. **Options:** 1. Only A is true. 2. Only B is true. 3. Both A and B are true. 4. Both A and B are false. **Analysis:** Firstly, we analyze the given statement \( Q(x) \): \[ Q(x): x + 1 > 2x \] Simplifying \( Q(x) \): \[ x + 1 > 2x \implies 1 > x \] Since \( x \) is a positive integer (from the domain definition), \( x \) must be greater than or equal to 1. Therefore, the inequality \( 1 > x \) is false for all positive integers \( x \). **Investigating Statement A:** Statement A posits that there exists an \( x \) such that \( Q(x) \) is true: \[ \exists x (x + 1 > 2x) \] Because \( Q(x) \) simplifies to \( 1 > x \), and there is no positive integer \( x \) for which this is true, Statement A is false. **Investigating Statement B:** Statement B posits that for every \( x \), the negation of \( Q(x) \) is true: \[ \forall x (\neg Q(x)) \] Since \( Q(x) \) is equivalent to \( 1 > x \), its negation is: \[ \neg Q(x): x \geq 1 \] This inequality \( x \geq 1 \) is true for all positive integers \( x \), hence Statement B is true. **Conclusion:** From the analysis, we can conclude that "Only B is true." Thus, the correct option is: \[ \boxed{\text{Only B is true.}} \]
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