Identify the correct translation into logical notation of the following…
2004
Identify the correct translation into logical notation of the following assertion.
"Some boys in the class are taller than all the girls" Note : taller(x,y) is true if x is taller than y.
- A.
(∃x) (boy(x) → (∀y) (girl(y) ∧ taller(x,y)))
- B.
(∃x) (boy(x) ∧ (∀y) (girl(y) ∧ taller(x,y)))
- C.
(∃x) (boy(x) → (∀y) (girl(y) → taller(x,y)))
- D.
(∃x) (boy(x) ∧ (∀y) (girl(y) → taller(x,y)))
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Correct answer: D
Correct translation: (∃x)(boy(x) ∧ (∀y)(girl(y) → taller(x,y)))
Explanation: There exists an individual x who is a boy, and for every individual y, if y is a girl then x is taller than y.
Why the outer conjunction is required: Using (∃x)(boy(x) → ...) would allow a non-boy to satisfy the existential by making the implication true vacuously, so it would not guarantee the existence of a boy with the property.
Why the inner implication is required: (∀y)(girl(y) → taller(x,y)) says for every y, being a girl is sufficient to require x be taller than y. Using (girl(y) ∧ taller(x,y)) inside the universal would incorrectly require that every object y is a girl and that x is taller than every object.
Common incorrect forms and why they fail:
Example incorrect formula: (∃x)(boy(x) → (∀y)(girl(y) → taller(x,y))).
Why it fails: The existential could be witnessed by an object that is not a boy, making the implication true without any boy being taller than all girls.
Example incorrect formula: (∃x)(boy(x) ∧ (∀y)(girl(y) ∧ taller(x,y))).
Why it fails: The universal conjunct (girl(y) ∧ taller(x,y)) asserts every object y is a girl and that x is taller than every object, which is stronger than the intended meaning.
Conclusion: The formula (∃x)(boy(x) ∧ (∀y)(girl(y) → taller(x,y))) precisely captures 'Some boys in the class are taller than all the girls.'
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