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The LCM and HCF of two polynomials \(p(x)\) and \(q(x)\) are \((x+a)(x^3-a^3)\) and \((x^2-ax+a^2)\), respectively. If \(p(x)=x^4+a^2x^2+a^4\), then what is \(q(x)\) equal to ?

This question was previously asked in
CDS 2 2026 Maths Question Paper (13-Sep-2026)
The correct answer is

\(x^2-a^2\)

To solve this problem, we need to use the relationship between the Least Common Multiple (LCM), Highest Common Factor (HCF), and the given polynomials.

We are given:

  • LCM of \(p(x)\) and \(q(x)\) is \((x+a)(x^3-a^3)\)
  • HCF of \(p(x)\) and \(q(x)\) is \((x^2-ax+a^2)\)
  • Polynomial \(p(x) = x^4 + a^2x^2 + a^4\)

And we need to find \(q(x)\).

Using the relationship:

\(LCM(p(x), q(x)) \cdot HCF(p(x), q(x)) = p(x) \cdot q(x).\)

Substitute the given values into the equation:

\(\left[(x+a)(x^3-a^3)\right] \cdot (x^2-ax+a^2) = (x^4 + a^2x^2 + a^4) \cdot q(x).\)

First, let's simplify \((x+a)(x^3-a^3)\):

  • Recognize that \(x^3-a^3 = (x-a)(x^2+ax+a^2)\) (using the difference of cubes).
  • So, \((x+a)(x^3-a^3) = (x+a)(x-a)(x^2+ax+a^2)\).

Thus, the LCM can be represented as:

\((x+a)(x-a)(x^2+ax+a^2).\)

Now the equation becomes:

\((x+a)(x-a)(x^2+ax+a^2)(x^2-ax+a^2) = (x^4 + a^2x^2 + a^4) \cdot q(x).\)

Simplify the left-hand side:

  • \((x+a)(x-a) = x^2-a^2\)
  • So, the left side is \((x^2-a^2)(x^2+ax+a^2)(x^2-ax+a^2)\).

With this simplification, we substitute back:

\((x^2-a^2) = q(x),\)

where the root of simplification and cancellation is observed. Thus, the correct polynomial \(q(x)\) that satisfies the equation is:

  • \(q(x) = x^2-a^2\)

Therefore, the option that matches is \(x^2-a^2\).

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Important Questions from LCM and HCF

  1. The HCF and LCM of two numbers are 12 and 72, respectively. If the ratio of the two numbers is 2 ∶ 3, then the larger of the two numbers is:

  2. Find the greatest number that will divide 43, 91 and 183 so as to leave the same remainder in each case.

  3. Joseph visits the club on every 5 th day, Harsh visits on every 24 th day, while Sumit visits on every 9 th day. If all three of them met at the club on a Sunday, then on which day will all three of them meet again?

  4. What is the least number which when divided by 12,20 and 24 leaves in each case a remainder of 8?

  5. Which of the following is a pair of co-primes?

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