What is the main reason for assuming (\sqrt{3}=\frac{p}{q}) in lowest form?
In lowest rational form, numerator and denominator are coprime. Later both becoming divisible by (3) gives the contradiction.
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SubjectsMathematics
√2 और √3 की अपरिमेयता का प्रमाण
In Class 9 Mathematics, this Number Systems topic explains how to prove that √2 and √3 are irrational numbers. Students use proof by contradiction: they assume a square root can be written as a fraction in lowest terms, then apply prime-factor and divisibility properties to show that the assumption leads to an impossibility. The lesson strengthens understanding of rational and irrational numbers, factors, parity, and the logic of mathematical proof, while helping learners present each step clearly and accurately.
TOPIC PRACTICE
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In lowest rational form, numerator and denominator are coprime. Later both becoming divisible by (3) gives the contradiction.
View question detailsIn contradiction method, the opposite of the statement is assumed first. So (\sqrt{2}) is assumed rational to get a contradiction.
View question detailsIf prime (3) divides the square, it also divides the number. Therefore (p) is written as (3k).
View question detailsFrom \(p^2=3q^2\), 3 divides \(p^2\), so 3 divides \(p\). Put \(p=3k\); then 3 also divides \(q\). This contradicts \(p/q\) being in lowest terms. Exam tip: lowest terms means \(p\) and \(q\) are coprime.
View question details(q^2) is divisible by (3), so (q) is also divisible by (3). This gives common factor (3) in both (p) and (q).
View question detailsFrom \(p^2=3q^2\), \(p^2\) is divisible by 3, so \(p\) is divisible by 3. Put \(p=3k\): then \(q^2=3k^2\), making \(q\) divisible by 3 too. This contradicts coprimality. Exam tip: if a prime divides \(p^2\), it divides \(p\).
View question detailsIf both \(p\) and \(q\) are divisible by \(3\), then they have \(3\) as a common factor. Hence, they cannot be coprime. In the irrationality proof of \(\sqrt{3}\), \(p\) and \(q\) are assumed to be coprime; showing that both are divisible by \(3\) gives the required contradiction. Therefore, option C is the wrong conclusion. Exam tip: coprime numbers always have HCF \(1\).
View question details50 is not a perfect square, and \(50=25\times2\), so \(\sqrt{50}=5\sqrt{2}\). Since \(\sqrt{2}\) is irrational, \(\sqrt{50}\) is also irrational. In contrast, 16, 36, and 64 are perfect squares. Exam tip: check for perfect squares first.
View question detailsFrom (q^2=3k^2), (q) is also divisible by (3). Both (p) and (q) divisible by (3) contradicts the coprime condition.
View question detailsTo prove √2 irrational, assume √2 = m/n, where m and n are coprime integers and n is non-zero. Squaring gives m² = 2n². The right side is even, so m² is even; consequently m is even. Write m = 2r. Substitution gives 4r² = 2n², hence n² = 2r², so n is also even. Thus m and n share a factor 2, contradicting the assumption that the fraction was in lowest terms. Option A presents this essential chain. Option B belongs to the analogous √3 argument, while C and D omit or contradict the required reasoning.
View question detailsFor the contradiction proof of √3, assume √3 = p/q in lowest terms, with q not equal to zero. Squaring gives p² = 3q². Therefore p² is divisible by the prime 3, and the prime-divisibility property implies that p is divisible by 3. Write p = 3k. Substituting and simplifying gives 3k² = q², so q², and hence q, is also divisible by 3. Both p and q then have a common factor 3, contradicting lowest terms. Option C states this correct chain. Option A uses the factor 2 and belongs to √2; the remaining options do not establish the contradiction.
View question detailsFrom q² = 3k², 3 divides q². Since 3 is prime, it must divide q. Also, p = 3k shows that 3 divides p, contradicting that p and q are coprime. Exam tip: if a prime divides a square, it divides the original number.
View question detailsNot being a perfect square only shows that \(\sqrt{3}\) is not an integer. In \(m^2=3n^2\), the power of 3 is even on the left but odd on the right, giving a contradiction. Exam tip: always assume lowest terms.
View question detailsA rational number is written in lowest fraction. The denominator is non-zero and numerator-denominator are coprime.
View question detailsIf \(m=2k\), then \(4k^2=2n^2\), so \(n^2=2k^2\) and \(n\) is also even. Thus both numbers have a common factor 2, contradicting coprimality. Exam tip: state this contradiction explicitly.
View question detailsSince \(p^2=3q^2\), \(p^2\) is divisible by 3, so \(p\) is divisible by 3. Put \(p=3k\); then \(q^2=3k^2\), making \(q\) divisible by 3 too. This contradicts coprimality. Exam tip: if a prime divides a square, it divides its root.
View question detailsThe governing fact is Euclid’s lemma for a prime: if a prime divides the square of an integer, it divides that integer itself. In the √3 proof, the equation p² = 3q² shows that 3 divides p². Because 3 is prime, it follows that 3 divides p, so p can be written as p = 3k. This is the required step and makes option B correct. The conclusion does not say that p is divisible by 2, equal to zero, or equal to q. After this step, substitution is used to show that q is also divisible by 3, producing the contradiction with lowest terms.
View question detailsThe final conclusion comes from contradiction with the original lowest-terms assumption. Suppose √2 = m/n, where m and n are coprime integers. Squaring gives m² = 2n², so m is even; writing m = 2r then leads to n² = 2r², making n even as well. Thus both numerator and denominator have the common factor 2, which contradicts their being coprime. The rational assumption must therefore be false, and √2 is irrational. Option A states the decisive basis. A terminating decimal is not the conclusion of this proof, m = n is never established, and n = 0 is excluded because a fraction cannot have a zero denominator.
View question detailsBoth being divisible by (3) breaks the coprime condition. Therefore (\sqrt{3}) cannot be rational.
View question detailsFrom (m^2=2n^2), first (m^2) and then (m) are even. Only after taking (m=2r), (n) is proved even.
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