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Question

What is the remainder when \(x^6\) is divided by \(x^2 + 1\)?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
-1

Remainder Calculation for \(x^6\) Divided by \(x^2+1\)

This problem asks us to find the remainder when the polynomial \(x^6\) is divided by the polynomial \(x^2 + 1\). We can solve this using algebraic manipulation, specifically by leveraging the properties of polynomial division and substitution.

Understanding Polynomial Division and Remainders

When we divide a polynomial \(P(x)\) by another polynomial \(D(x)\), we get a quotient \(Q(x)\) and a remainder \(R(x)\) such that:

\(P(x) = D(x) \cdot Q(x) + R(x)\)

The degree of the remainder \(R(x)\) must be strictly less than the degree of the divisor \(D(x)\). In our case:

  • \(P(x) = x^6\) (the dividend)
  • \(D(x) = x^2 + 1\) (the divisor)

Since the degree of the divisor \(D(x)\) is 2 (the highest power of \(x\) is \(x^2\)), the remainder \(R(x)\) must have a degree less than 2. This means the remainder will be of the form \(ax + b\), where \(a\) and \(b\) are constants.

Method: Using Substitution

A quick way to find the remainder is to use the relationship derived from the divisor.

  1. Set the divisor to zero to find the values of \(x\) that make it zero: \(x^2 + 1 = 0\)
  2. Solve for \(x^2\): \(x^2 = -1\)
  3. Now, we rewrite the dividend \(x^6\) in terms of \(x^2\): \(x^6 = (x^2)^3\)
  4. Substitute the value \(x^2 = -1\) into the expression for \(x^6\): \(x^6 = (-1)^3\)
  5. Calculate the result: \((-1)^3 = -1 \times -1 \times -1 = -1\)

This value, -1, represents the remainder when \(x^6\) is divided by \(x^2+1\). Because this result is a constant, it fits the requirement for the remainder \(R(x)\) (degree less than 2).

Step-by-Step Algebraic Approach

Alternatively, we can use algebraic steps:

  1. Start with the dividend \(x^6\).
  2. We know \(x^2 = -1\) implies \(x^2+1=0\). We want to express \(x^6\) using terms of \(x^2+1\).
  3. Rewrite \(x^6\) as \((x^2)^3\).
  4. Substitute \(x^2 = -1 + (x^2+1)\): \(x^6 = (x^2)^3 = (-1 + (x^2+1))^3\)
  5. Expanding this using the binomial theorem would be complex. Instead, let's directly use \(x^2 \equiv -1 \pmod{x^2+1}\).
  6. \(x^6 = (x^2)^3 \equiv (-1)^3 \pmod{x^2+1}\)
  7. \(x^6 \equiv -1 \pmod{x^2+1}\)

This shows that \(x^6\) leaves a remainder of \(-1\) when divided by \(x^2+1\).

Conclusion

Both methods confirm that the remainder when \(x^6\) is divided by \(x^2 + 1\) is \(-1\).

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Similar Questions

  1. Let \(p(x)\) be a polynomial. When \(p(x)\) is divided by \((x-1)\), it leaves 2 as the remainder. When \(p(x)\) is divided by \((x-2)\), it leaves 1 as the remainder. What is the remainder when \(p(x)\) is divided by \((x - 1)(x-2)\)?
  2. \((x+2)\) is a factor of which one of the following?
  3. What is the HCF of the polynomials x⁸ + x⁴ + 1 and x⁴ + x² + 1?

  4. If 2 is a zero of the polynomial \(p(x) = x^3 + 3x^2 - 6x - a\), then what is the sum of the squares of the other zeros of the polynomial?
  5. Suppose \(p(x) = x^4 + a_3x^3 + a_2x^2 + a_1x + a_0\) and \(q(x) = x^4 + b_3x^3 + b_2x^2 + b_1x + b_0\) are the polynomials. If \(\alpha, \beta, \gamma, \delta\) are zeros of \(p(x)\) and \(\alpha, \beta, \gamma, \lambda\) are zeros of \(q(x)\), then what is \(\frac{p(x) - q(x)}{(x - \alpha) (x - \beta) (x - \gamma)}\) equal to ?
  6. Consider the following in respect of the polynomial \(x^{4k} + x^{4k+2} + x^{4k+4} + x^{4k+6}\):
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    Which of the statements given above is/are correct?
  7. If \(x^3 + px^2 + qx + r\) is an integer for all integral values of \(x\), then consider the following statements :
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  8. Which of the following expressions can divide both the polynomials \(x^3+2x^2-5x+2\) and \(x^3+4x^2+x-6\) exactly ?
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    II. \(x+1\)
    III. \(x+2\)
    Select the correct answer using the code given below :
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Important Questions from Polynomials

  1. If y 2= y + 7, then what is the value of y 3?

  2. Factorize x 2- y 2- 9z 2+ 6yz

  3. If one of the zeros of the polynomial x 3+ ax 2+ bx + c is  - 1, then the product of other two zeros is equal to :

  4. If a(a + b + c) 2 = 1792; b(a + b + c) 2 = 1536; c(a + b + c) 2 = 768, then what will be the value of b?

  5. If x = 3 so, what is the value of x 2 + 2x + 5 ?

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