Newton + Telescope

Algebra Level 4

If x 2 + x 1 = 0 x^2+x-1=0 then evaluate r = 0 2016 ( 2017 r 2 ) { r 2 } x r \large \sum_{r=0}^{2016} \Big (\frac {2017-r}{2}\Big )^{\lceil \{\frac {r}{2}\} \rceil}\cdot x^r .

Notation : { } \{ \cdot \} denotes the fractional part function .


The answer is 1009.

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1 solution

Rajen Kapur
Apr 27, 2016

x 2 + x 1 = 0 x^2+x-1=0 can be generalized to x n + x n 1 = x n 2 x^n+x^{n-1}=x^{n-2} which is used repetitively. Now let us write the polynomial with r taking values down from 2016, 2015, 2014, . . . ., 3, 2, 1, 0. x 2016 + x 2015 + x 2014 + 2 x 2013 + x 2012 + 3 x 2011 + x 2010 + 4 x 2009 + . . . + 1008 x + 1 x^{2016}+x^{2015}+x^{2014}+2x^{2013}+x^{2012}+3x^{2011}+x^{2010}+4x^{2009}+ . . .+1008x+1 2 x 2014 + 2 x 2013 + x 2012 + 3 x 2011 + x 2010 + 4 x 2009 + . . + 1008 x + 1 \rightarrow2x^{2014}+2x^{2013}+x^{2012}+3x^{2011}+x^{2010}+4x^{2009}+ . .+1008x+1 3 x 2012 + 3 x 2011 + x 2010 + 4 x 2009 + . . + 1008 x + 1. \rightarrow 3x^{2012}+3x^{2011}+x^{2010}+4x^{2009}+ . . +1008x+1. Proceeding this way one can reach the answer.

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