Polynomial Remainder Theorem

XAT 2013 · Quantitative Ability & Data Interpretation · Hard · Algebra

Consider a function f(x)=x4+x3+x2+x+1f(x) = x^4 + x^3 + x^2 + x + 1, where x is a positive integer greater than 1. What will be the remainder if f(x5)f(x^5) is divided by f(x)f(x)?

  1. A.

    5

  2. B.

    6

  3. C.

    7

  4. D.

    a monomial in x

  5. E.

    a polynomial in x

Answer

A

Explanation

f(x)=x5−1x−1  ⟹  x5≡1(modf(x))f(x) = \frac{x^5 - 1}{x - 1} \implies x^5 \equiv 1 \pmod{f(x)}. f(x5)=(x5)4+(x5)3+(x5)2+(x5)+1f(x^5) = (x^5)^4 + (x^5)^3 + (x^5)^2 + (x^5) + 1. Since x5≡1(modf(x))x^5 \equiv 1 \pmod{f(x)}, f(x5)≡14+13+12+1+1=5(modf(x))f(x^5) \equiv 1^4 + 1^3 + 1^2 + 1 + 1 = 5 \pmod{f(x)}. So remainder is 5.

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