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”
- A.
\(\exists x (\text{real}(x) \lor \text{rational}(x))\) - B.
\(\forall x (\text{real}(x) \to \text{rational}(x))\) - C.
\(\exists x (\text{real}(x) \wedge \text{rational}(x))\) - D.
\(\exists x (\text{rational}(x) \to \text{real}(x))\)
Attempted by 121 students.
Show answer & explanation
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).
A video solution is available for this question — log in and enroll to watch it.