Let 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 . Define a n = k = 1 ∑ 4 k x k n . Compute
a 2 0 1 7 + 2 a 2 0 1 5 a 2 0 1 8 + 3 a 2 0 1 6 + 5 a 2 0 1 4 .
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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 ;)
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This was a nice question and I wanted to explain every bit of it in my solution.... ;-)... BTW Thanks...
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Very well done indeed!
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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...
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Since 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 2 4 + 2 x 2 3 + 3 x 2 2 + 4 x 2 + 5 = 0 . . . . ( 2 ) x 3 4 + 2 x 3 3 + 3 x 3 2 + 4 x 3 + 5 = 0 . . . . ( 3 ) x 4 4 + 2 x 4 3 + 3 x 4 2 + 4 x 4 + 5 = 0 . . . . ( 4 )
Multiplying ( 1 ) by x n , ( 2 ) by 2 x n , ( 3 ) by 3 x n and ( 4 ) by 4 x 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
⟹ a n + 4 + 2 a n + 3 + 3 a n + 2 + 4 a n + 1 + 5 a n = 0 ⟹ a n + 3 + 2 a n + 1 a n + 4 + 3 a n + 2 + 5 a n = − 2 Put n = 2 0 1 4 .
⟹ a 2 0 1 7 + 2 a 2 0 1 5 a 2 0 1 8 + 3 a 2 0 1 6 + 5 a 2 0 1 4 = − 2