Are Recurrence Relations Related?

Algebra Level 4

Let x 1 , x 2 , x 3 , x 4 x_1, x_2, x_3, x_4 be the roots of the polynomial x 4 + 2 x 3 + 3 x 2 + 4 x + 5 x^4+2x^3+3x^2+4x+5 . Define a n = k = 1 4 k x k n \displaystyle a_n=\sum_{k=1}^{4}kx_k^{n} . Compute

a 2018 + 3 a 2016 + 5 a 2014 a 2017 + 2 a 2015 . \dfrac{a_{2018}+3a_{2016}+5a_{2014}}{a_{2017}+2a_{2015}} \; .


Inspiration .


The answer is -2.

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1 solution

Rishabh Jain
Mar 25, 2016

Since x 1 , x 2 , x 3 , x 4 x_1,x_2,x_3,x_4 are roots of the given equation. Therefore: x 1 4 + 2 x 1 3 + 3 x 1 2 + 4 x 1 + 5 = 0 . . . . ( 1 ) x_1^4+2x_1^3+3x_1^2+4x_1+5=0\color{#3D99F6}{....(1)} x 2 4 + 2 x 2 3 + 3 x 2 2 + 4 x 2 + 5 = 0 . . . . ( 2 ) x_2^4+2x_2^3+3x_2^2+4x_2+5=0\color{#3D99F6}{....(2)} x 3 4 + 2 x 3 3 + 3 x 3 2 + 4 x 3 + 5 = 0 . . . . ( 3 ) x_3^4+2x_3^3+3x_3^2+4x_3+5=0\color{#3D99F6}{....(3)} x 4 4 + 2 x 4 3 + 3 x 4 2 + 4 x 4 + 5 = 0 . . . . ( 4 ) x_4^4+2x_4^3+3x_4^2+4x_4+5=0\color{#3D99F6}{....(4)}

Multiplying ( 1 ) \color{#3D99F6}{(1)} by x n x^n , ( 2 ) \color{#3D99F6}{(2)} by 2 x n 2x^n , ( 3 ) \color{#3D99F6}{(3)} by 3 x n 3x^n and ( 4 ) \color{#3D99F6}{(4)} by 4 x n 4x^n and adding all gives :

k = 1 4 k x k n + 4 + 2 k = 1 4 k x k n + 3 + 3 k = 1 4 k x k n + 2 + 4 k = 1 4 k x k n + 1 + 5 k = 1 4 k x k n = 0 \sum_{k=1}^{4}kx_k^{n+4}+2\sum_{k=1}^{4}kx_k^{n+3}+3\sum_{k=1}^{4}kx_k^{n+2}+4\sum_{k=1}^{4}kx_k^{n+1}+5\sum_{k=1}^{4}kx_k^{n}=0

a n + 4 + 2 a n + 3 + 3 a n + 2 + 4 a n + 1 + 5 a n = 0 \implies a_{n+4}+2a_{n+3}+3a_{n+2}+4a_{n+1}+5a_n=0 a n + 4 + 3 a n + 2 + 5 a n a n + 3 + 2 a n + 1 = 2 \implies \dfrac{a_{n+4}+3a_{n+2}+5a_n}{a_{n+3}+2a_{n+1}}=-2 Put n = 2014 n=2014 .

a 2018 + 3 a 2016 + 5 a 2014 a 2017 + 2 a 2015 = 2 \implies \dfrac{a_{2018}+3a_{2016}+5a_{2014}}{a_{2017}+2a_{2015}}=\boxed{-2}

Yes, exactly! Very nicely explained! (+1) I was afraid that I would have to write a three-line solution... but now I don't have to ;)

Otto Bretscher - 5 years, 2 months ago

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This was a nice question and I wanted to explain every bit of it in my solution.... ;-)... BTW Thanks...

Rishabh Jain - 5 years, 2 months ago

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Very well done indeed!

Otto Bretscher - 5 years, 2 months ago

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@Otto Bretscher Thanks.... :-) This was the the first time I used recursion in question like this... I saw it first in your solution on Mohit's problem.... Brilliant job...

Rishabh Jain - 5 years, 2 months ago

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