Justify (argue, prove, ...) that the following formula is valid Vx (A(x)→B(x)) → (Vx A(x)→V¢B(x))

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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**Explanation of Formula Validity**

We are tasked with proving the validity of the following logical formula:

\[
\forall x \ (A(x) \rightarrow B(x)) \rightarrow (\forall x \ A(x) \rightarrow \forall x \ B(x))
\]

**Explanation:**

1. **Understanding the Formula:**
   - The expression consists of an implication (indicated by \(\rightarrow\)) between two universally quantified statements (indicated by \(\forall x\)).
   - The left part of the implication: \(\forall x \ (A(x) \rightarrow B(x))\), asserts that for every element \(x\), if \(A(x)\) is true, then \(B(x)\) is true.
   - The right part of the implication: \(\forall x \ A(x) \rightarrow \forall x \ B(x)\), suggests that if \(A(x)\) is true for all \(x\), then \(B(x)\) must also be true for all \(x\).

2. **Proof Outline:**
   - Assume the premise \(\forall x \ (A(x) \rightarrow B(x))\) is true.
   - We need to show that from this assumption, \(\forall x \ A(x) \rightarrow \forall x \ B(x)\) follows.
   - Assume \(\forall x \ A(x)\) (for each \(x\), \(A(x)\) is true). Given this assumption and the premise, it follows that \(B(x)\) must also be true for each such \(x\) as \(A(x) \rightarrow B(x)\).
   - Therefore, \(\forall x \ B(x)\) holds.

In conclusion, the formula is valid as the premise logically ensures the conclusion through universal instantiation and modus ponens.
Transcribed Image Text:**Explanation of Formula Validity** We are tasked with proving the validity of the following logical formula: \[ \forall x \ (A(x) \rightarrow B(x)) \rightarrow (\forall x \ A(x) \rightarrow \forall x \ B(x)) \] **Explanation:** 1. **Understanding the Formula:** - The expression consists of an implication (indicated by \(\rightarrow\)) between two universally quantified statements (indicated by \(\forall x\)). - The left part of the implication: \(\forall x \ (A(x) \rightarrow B(x))\), asserts that for every element \(x\), if \(A(x)\) is true, then \(B(x)\) is true. - The right part of the implication: \(\forall x \ A(x) \rightarrow \forall x \ B(x)\), suggests that if \(A(x)\) is true for all \(x\), then \(B(x)\) must also be true for all \(x\). 2. **Proof Outline:** - Assume the premise \(\forall x \ (A(x) \rightarrow B(x))\) is true. - We need to show that from this assumption, \(\forall x \ A(x) \rightarrow \forall x \ B(x)\) follows. - Assume \(\forall x \ A(x)\) (for each \(x\), \(A(x)\) is true). Given this assumption and the premise, it follows that \(B(x)\) must also be true for each such \(x\) as \(A(x) \rightarrow B(x)\). - Therefore, \(\forall x \ B(x)\) holds. In conclusion, the formula is valid as the premise logically ensures the conclusion through universal instantiation and modus ponens.
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