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Question

What is HCF of
\(a^4+2a^3+3a^2+2a+1\) and \(a^6-2a^3+1\)?

This question was previously asked in
CDS 2 2024 Maths Question Paper (01-Sep-2024)
The correct answer is
\((a^2+a+1)^2\)

Understanding the Problem: Finding the HCF of Polynomials

The question asks us to find the Highest Common Factor (HCF), also known as the Greatest Common Divisor (GCD), of two given polynomial expressions:

  • Polynomial 1: \( P(a) = a^4+2a^3+3a^2+2a+1 \)
  • Polynomial 2: \( Q(a) = a^6-2a^3+1 \)

To find the HCF, we need to factorize both polynomials completely and identify the common factors.

Step 1: Factorizing the Polynomial \( Q(a) = a^6-2a^3+1 \)

The polynomial \( Q(a) \) can be seen as a quadratic expression in terms of \( a^3 \). Let \( x = a^3 \). Then the expression becomes:

\( x^2 - 2x + 1 \)

This is a perfect square trinomial, which factors as:

\( (x-1)^2 \)

Now, substitute \( x = a^3 \) back into the factored expression:

\( (a^3 - 1)^2 \)

We know the difference of cubes formula: \( a^3 - 1 = (a-1)(a^2+a+1) \). Substituting this:

\( [(a-1)(a^2+a+1)]^2 \)

Expanding the square, we get the complete factorization of \( Q(a) \):

\( Q(a) = (a-1)^2 (a^2+a+1)^2 \)

Step 2: Factorizing the Polynomial \( P(a) = a^4+2a^3+3a^2+2a+1 \)

The polynomial \( P(a) \) is a reciprocal polynomial because its coefficients are symmetric (1, 2, 3, 2, 1). We can factor it by dividing by \( a^2 \) (assuming \( a \neq 0 \)) and rearranging:

\( \frac{P(a)}{a^2} = \frac{a^4}{a^2} + \frac{2a^3}{a^2} + \frac{3a^2}{a^2} + \frac{2a}{a^2} + \frac{1}{a^2} \) \( = a^2 + 2a + 3 + \frac{2}{a} + \frac{1}{a^2} \)

Group the terms:

\( = \left( a^2 + \frac{1}{a^2} \right) + 2\left( a + \frac{1}{a} \right) + 3 \)

Let \( y = a + \frac{1}{a} \). Then \( y^2 = \left( a + \frac{1}{a} \right)^2 = a^2 + 2(a)\left(\frac{1}{a}\right) + \frac{1}{a^2} = a^2 + 2 + \frac{1}{a^2} \). Therefore, \( a^2 + \frac{1}{a^2} = y^2 - 2 \).

Substitute \( y \) and \( y^2 - 2 \) into the expression:

\( = (y^2 - 2) + 2y + 3 \) \( = y^2 + 2y + 1 \)

This is a perfect square:

\( = (y+1)^2 \)

Now, substitute \( y = a + \frac{1}{a} \) back:

\( = \left( a + \frac{1}{a} + 1 \right)^2 \)

To get \( P(a) \), multiply by \( a^2 \):

\( P(a) = a^2 \left( a + \frac{1}{a} + 1 \right)^2 \) \( P(a) = \left[ a \left( a + \frac{1}{a} + 1 \right) \right]^2 \) \( P(a) = (a^2 + 1 + a)^2 \) \( P(a) = (a^2+a+1)^2 \)

Step 3: Identifying the Highest Common Factor (HCF)

We have the factorizations:

  • \( P(a) = (a^2+a+1)^2 \)
  • \( Q(a) = (a-1)^2 (a^2+a+1)^2 \)

To find the HCF, we look for the common factors raised to the lowest power they appear in either factorization. The common factor is \( (a^2+a+1) \). The lowest power it appears with is 2 in both factorizations.

Therefore, the HCF is:

\( \text{HCF}(P(a), Q(a)) = (a^2+a+1)^2 \)

Conclusion

Comparing our result with the given options, the HCF of the two polynomials is \( (a^2+a+1)^2 \).

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