Polynomials going crazy!

Algebra Level 5

The polynomial f ( x ) = x 2007 + 17 x 2006 + 1 f(x) = x^{2007} + 17x^{2006} + 1 has distinct roots r 1 , r 2 , , r 2007 r_1, r_2,\ldots, r_{2007} . A polynomial P P of degree 2007 has the property that P ( r j + 1 r j ) = 0 P \left( r_j + \dfrac1{r_j} \right) = 0 for j = 1 , 2 , , 2007 j = 1,2,\ldots,2007 .

Compute 259 17 × P ( 1 ) P ( 1 ) \dfrac{259}{17} \times \dfrac{P(1)}{P(-1)} .


The answer is 17.

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2 solutions

Arturo Presa
Jun 12, 2016

We can see that none of the roots of f ( x ) f(x) is zero. So we can form a polynomial whose roots are the numbers 1 r j \frac{1}{r_j} for j = 1 , 2 , 3 , . . . , 2007 j=1, 2, 3, ..., 2007 and with 1 as leading coefficient just by considering g ( x ) = x 2007 f ( 1 x ) g(x)=x^{2007} f(\frac{1}{x}) , and, additionally, g ( x ) = j = 1 2007 ( x 1 r j ) . g(x)=\prod_{j=1}^{2007}(x-\frac{1}{r_j}).

Since P ( x ) = c j = 1 2007 ( x r j 1 r j ) P(x)=c\prod_{j=1}^{2007}(x-r_j-\frac{1}{r_j}) where c c is a constant then, P ( x + 1 x ) = c j = 1 2007 ( x + 1 x r j 1 r j ) = c j = 1 2007 ( x r j ) ( x 1 r j ) x = c x 2007 f ( x ) g ( x ) = c f ( x ) f ( 1 x ) . \begin{aligned} P(x+\frac{1}{x})&=c\prod_{j=1}^{2007}(x+\frac{1}{x}-r_j-\frac{1}{r_j})\\ &=c \prod_{j=1}^{2007}\frac{(x-r_j)(x-\frac{1}{r_j})}{x}\\ &=c x^{-2007}f(x)g(x)\\ &=c f(x)f(\frac{1}{x})\end{aligned}.

Now, since e i π 3 + 1 e i π 3 = 1 , e^{i\frac{\pi}{3}}+\frac{1}{e^{i\frac{\pi}{3}}}=1, e i 2 π 3 + 1 e i 2 π 3 = 1 , e^{i\frac{2\pi}{3}}+\frac{1}{e^{i\frac{2\pi}{3}}}=-1, and using the equality above, we obtain that 259 17 × P ( 1 ) P ( 1 ) = 259 17 × P ( e i π 3 + 1 e i π 3 ) P ( e i 2 π 3 + 1 e i 2 π 3 ) = 259 17 × f ( i π 3 ) f ( 1 e i π 3 ) f ( e i 2 π 3 ) f ( 1 e i 2 π 3 ) \frac{259}{17}\times \frac{P(1)}{P(-1)}=\frac{259}{17}\times \frac{P\left(e^{i\frac{\pi}{3}}+\frac{1}{e^{i\frac{\pi}{3}}}\right)}{P\left(e^{i\frac{2\pi}{3}}+\frac{1}{e^{i\frac{2\pi}{3}}}\right)}=\frac{259}{17}\times\frac{f(i\frac{\pi}{3})f\left(\frac{1}{e^{i\frac{\pi}{3}}}\right)}{f(e^{i\frac{2\pi}{3}})f\left(\frac{1}{e^{i\frac{2\pi}{3}}}\right)} = 259 17 × 1 7 2 259 = 17 . =\frac{259}{17}\times \frac{17^2}{259}=\boxed{17}.

Nipun Gupta
Dec 28, 2015

same here. nice problem.

Aareyan Manzoor - 5 years, 5 months ago

i was doing same but didnt notice substitution of w

Dev Sharma - 5 years, 5 months ago

Can I ask its source?

Dev Sharma - 5 years, 5 months ago

Log in to reply

Masterstoke.....!!! :D

Nipun Gupta - 5 years, 5 months ago

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