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In proving the irrationality of
ext{\(\sqrt{3}\)}
, Ravi assumes that
ext{\(\sqrt{3}=p/q\)}
, where
ext{\(p\)}
and
ext{\(q\)}
are coprime. From
ext{\(p^2=3q^2\)}
, he says that only
ext{\(p\)}
is divisible by 3 and nothing can be concluded about
ext{\(q\)}
. Which fact corrects his error?

Answer and explanation

Correct answer: If 3 divides \(q^2\), then 3 also divides \(q\).

Since 3 is prime, \(p^2=3q^2\) implies that 3 divides \(p\). Put \(p=3k\); then \(q^2=3k^2\), so 3 divides \(q\) as well. This contradicts that \(p\) and \(q\) are coprime. Exam tip: if a prime divides a square, it divides the number itself.

Related tags

Number SystemsIrrational NumbersProof By ContradictionSquare Root 3Prime DivisibilityProof Of Irrationality Of Square Root 2 And Square Root 3MathematicsClass 9 Mcq

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