If \(𝑓(𝑥)=𝑥\) is my friend, and \(𝑝(𝑥)=𝑥\) is perfect, then correct…

2020

If \(𝑓(𝑥)=𝑥\) is my friend, and \(𝑝(𝑥)=𝑥\) is perfect, then correct logical translation of the statement “some of my friends are not perfect” is ______

  1. A.

    \(\forall _x (f(x) \wedge \neg p(x))\)

  2. B.

    \(\exists _x (f(x) \wedge \neg p(x))\)

  3. C.

    \(\neg (f(x) \wedge \neg p(x))\)

  4. D.

    \(\exists _x (\neg f(x) \wedge \neg p(x))\)

Attempted by 60 students.

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Correct answer: B

Correct translation: ∃x (f(x) ∧ ¬p(x))

  • Interpret "some of my friends" as "there exists an x such that f(x) is true": this gives the existential quantifier ∃x and the condition f(x).

  • Interpret "are not perfect" as ¬p(x), applied to the same x.

  • Combine both conditions for the same x with conjunction inside the scope of the existential quantifier: ∃x (f(x) ∧ ¬p(x)).

Common incorrect alternatives and why they fail:

  • ∀x (f(x) ∧ ¬p(x)) — This claims every object is a friend and not perfect, which is much stronger than "some".

  • ¬(f(x) ∧ ¬p(x)) — As written this lacks a quantifier (so is not a well-formed sentence about 'some') and, if universally quantified, would mean for each x it is not the case that x is a friend and not perfect, which contradicts the intended existential claim.

  • ∃x (¬f(x) ∧ ¬p(x)) — This finds someone who is not a friend and not perfect, which does not assert anything about the perfection status of friends.

Final answer: ∃x (f(x) ∧ ¬p(x))

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