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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 1 2026 Maths Question Paper (12-Apr-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 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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