In a proof that \(\sqrt{3}\) is irrational, a student assumes \(\sqrt{3}=\frac{p}{q}\), where \(p\) and \(q\) are coprime, and obtains \(p^2=3q^2\). The student directly writes that \(q\) is divisible by 3. Which statement is needed to make the reasoning valid?

Correct answer: If \(3\mid p^2\), then \(3\mid p\); substituting \(p=3k\) then gives \(3\mid q\).

From \(p^2=3q^2\), we get \(3\mid p^2\). Since 3 is prime, \(3\mid p\); put \(p=3k\) to obtain \(q^2=3k^2\), so \(3\mid q\) too. This contradicts coprimality. Exam tip: state the prime-divides-square rule first.