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Question

If the HCF of p and q (p > q) is G, then which of the following statements is/are correct ?
I. HCF of p and (p + q) is G
II. HCF of p, (p - q) is G
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This question was previously asked in
CDS 1 2026 Maths Question Paper (12-Apr-2026)
The correct answer is
Both I and II

Understanding HCF Properties

We are given that the Highest Common Factor (HCF) of \(p\) and \(q\) is \(G\), where \(p > q\). This means we can express \(p\) and \(q\) as:

  • \(p = Gx\)
  • \(q = Gy\)

Here, \(x\) and \(y\) are coprime integers (their HCF is 1), and since \(p > q\), we know \(x > y\).

Analyzing Statement I

Statement I checks the HCF of \(p\) and \((p + q)\).

Let's find \((p + q)\): \(p + q = Gx + Gy = G(x + y)\)

Now, we find the HCF of \(p\) and \((p + q)\): \(HCF(p, p + q) = HCF(Gx, G(x + y))\)

Using the property \(HCF(ka, kb) = k \times HCF(a, b)\), we get: \(HCF(p, p + q) = G \times HCF(x, x + y)\)

We know that \(HCF(a, b) = HCF(a, b - a)\). Applying this: \(HCF(x, x + y) = HCF(x, (x + y) - x) = HCF(x, y)\)

Since \(x\) and \(y\) are coprime, \(HCF(x, y) = 1\). Therefore: \(HCF(p, p + q) = G \times 1 = G\)

So, Statement I is correct.

Analyzing Statement II

Statement II checks the HCF of \(p\) and \((p - q)\).

Let's find \((p - q)\): \(p - q = Gx - Gy = G(x - y)\)

Now, we find the HCF of \(p\) and \((p - q)\): \(HCF(p, p - q) = HCF(Gx, G(x - y))\)

Applying the property \(HCF(ka, kb) = k \times HCF(a, b)\): \(HCF(p, p - q) = G \times HCF(x, x - y)\)

Using the property \(HCF(a, b) = HCF(a, a - b)\) or similar difference properties: \(HCF(x, x - y) = HCF(x - (x - y), x - y) = HCF(y, x - y)\)

Alternatively, \(HCF(x, x - y) = HCF(x, y)\) because any common factor of \(x\) and \(x-y\) must also divide their difference, which is \(y\). Since \(x\) and \(y\) are coprime, \(HCF(x, y) = 1\). Therefore: \(HCF(x, x - y) = 1\)

So, \(HCF(p, p - q) = G \times 1 = G\).

Statement II is also correct.

Conclusion

Both Statement I and Statement II are correct. Thus, the correct option includes both.

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Important Questions from LCM and HCF

  1. The greatest three-digit number which is divisible by 14, 28, and 42 is:

  2. What is the greatest number that will divide 209 and 347 leaving remainder 5 and 7 respectively?

  3. A number is three times another number and their HCF is 8. What is the sum of the squares of the numbers?

  4. The HCF of 2091, 3485 and 4879 is x. The sum of the digits of x is:

  5. If three numbers are in ratio of 3 : 5 : 7 and their LCM is 2415, what is the difference between the second number and the first number?

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