Outrageous Quotient And Remainder

Algebra Level 5

Let P = 200 8 2007 2008 P=2008^{2007} - 2008 and Q = 200 8 2 + 2009 Q=2008^{2} + 2009 .
What is the remainder when P P is divided by Q Q ?


Source : NMTC.


The answer is 4032066.

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3 solutions

Salz City
Aug 20, 2016

Let R R and S S be the next polinomials:

R ( x ) = x ( x 2006 1 ) R(x) = x(x^{2006} - 1) S ( x ) = x 2 + x + 1 S(x) = x^{2} + x +1

We use this polinomials to solve this problem: P = 200 8 2007 2008 = 2008 ( 200 8 2006 1 ) P = 2008^{2007} - 2008 = 2008 \cdot (2008^{2006} -1) so P = R ( 2008 ) P = R(2008) Q = 200 8 2 + 2009 = 200 8 2 + 2008 + 1 Q = 2008^{2} + 2009 = 2008^{2} + 2008 +1 so Q = S ( 2008 ) Q = S(2008)

We know that for every natural number n n ,

x n 1 = ( x 1 ) ( x n 1 + x n 2 + + x 2 + x + 1 ) x^{n} - 1 = (x - 1) \cdot (x^{n-1} + x^{n-2} + \cdots + x^{2} + x + 1)

Applying this relation to R ( x ) R(x) we have,

R ( X ) = x ( x 1 ) ( x 2005 + x 2004 + + x 2 + x + 1 ) = ( x 1 ) ( x 2006 + x 2005 + + x 3 + x 2 + x ) = ( x 1 ) [ ( x 2004 + x 2001 + + x 6 + x 3 + 1 ) ( x 2 + x + 1 ) 1 ] ( 1 x ) ( m o d S ( x ) ) ( x 2 + 2 ) ( m o d S ( x ) ) \begin{aligned} R(X) &= x \cdot (x - 1) \cdot ( x^{2005} +x^{2004} + \cdots + x^{2} + x + 1) \\ &= (x - 1) \cdot ( x^{2006} + x^{2005} + \cdots + x^{3} + x^{2} + x) \\ &= (x - 1) \cdot [(x^{2004} + x^{2001} + \cdots + x^{6} + x^{3} + 1) \cdot (x^{2} + x + 1) - 1] \\ &\equiv (1 - x) \pmod{S(x)} \\ &\equiv (x^{2} + 2) \pmod{S(x)} \\ \end{aligned}

Then

P = R ( 2008 ) ( 200 8 2 + 2 ) ( m o d S ( 2008 ) ) 4032066 ( m o d Q ) \begin{aligned} P &= R(2008) \\ &\equiv (2008^{2} + 2)\pmod{S(2008)} \\ &\equiv \color{#D61F06} {4032066}\pmod{Q} \\ \end{aligned}

So, the answer is 4032066 \fbox{ 4032066 }

notice that P is in the form x 2007 x^{2007} -x and Q is in the form x 2 x^{2} +x+1

let f(x)= x 2007 x^{2007} -x g(x)= x 2 x^{2} +x+1

f(x)=g(x)*q(x)+R

Let g(x) be 0.

Then f(x)=R

x 2 x^{2} +x+1=0

x 3 x^{3} =1

putting the value in f(x)

f(x)= x 2 x^2 +2 =2008*2008+2 =4032066

R=4032066

A Steven Kusuman
Aug 28, 2016

using multiple subtractions , notice that a mod b = (a-b) mod b finally got the answer ^_^ , great problems

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