What is the correct translation of the following statement into mathematical…

2012

What is the correct translation of the following statement into mathematical logic?

“Some real numbers are rational”

  1. A.

    \(\exists x (\text{real}(x) \lor \text{rational}(x))\)

  2. B.

    \(\forall x (\text{real}(x) \to \text{rational}(x))\)

  3. C.

    \(\exists x (\text{real}(x) \wedge \text{rational}(x))\)

  4. D.

    \(\exists x (\text{rational}(x) \to \text{real}(x))\)

Attempted by 121 students.

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Correct answer: C

Correct translation: There exists at least one x that is both real and rational. Symbolically: ∃x (real(x) ∧ rational(x)).

Reasoning:

  • The formula ∃x (real(x) ∨ rational(x)) says there exists an x that is real or rational. It does not require the same x to be both real and rational, so it does not express "some real numbers are rational."

  • The formula ∀x (real(x) → rational(x)) says every real number is rational. That is a universal (all) claim and is stronger than the intended existential (some) claim.

  • The formula ∃x (real(x) ∧ rational(x)) correctly asserts the existence of an x that has both properties, matching the English statement.

  • The formula ∃x (rational(x) → real(x)) is true for many x simply because the implication is true when x is not rational; it therefore does not capture that some x are both real and rational.

Note: If the domain of discourse is already all real numbers, the predicate real(x) is true for every x and the correct translation can be simplified to: ∃x rational(x).

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