An algebra problem by Ojas Singh Malhi

Algebra Level 3

f ( x ) f(x) is a 5 t h 5th degree polynomial with leading coefficient 2009 2009 . If f ( 1 ) = 1 , f ( 2 ) = 3 , f ( 3 ) = 5 , f ( 4 ) = 7 , f ( 5 ) = 9 f(1)=1, f(2)=3, f(3)=5, f(4)=7, f(5)=9
Find f ( 6 ) f(6)


The answer is 241091.

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1 solution

Ojas Singh Malhi
Nov 13, 2017

Observe that the outputs of the polynomial follow the pattern 2 x 1 2x-1 . But this isn't the answer to the question as it requires a 5 t h 5th degree polynomial.
Therefore, we need a polynomial such that x = 1 , 2 , 3 , 4 , 5 x=1,2,3,4,5 , gives f ( x ) = 2 x 1 f(x)=2x-1 .
Clearly, f ( x ) = 2009 ( x 1 ) ( x 2 ) ( x 3 ) ( x 4 ) ( x 5 ) + 2 x 1 f(x)=2009(x-1)(x-2)(x-3)(x-4)(x-5)+2x-1 fulfils our requirements ( 5 t h 5th degree polynomial with leading coefficient 2009 2009 ).
Now, f ( 6 ) = 2009 5 4 3 2 1 + 11 f(6)=2009*5*4*3*2*1+11
f ( 6 ) = 241091 f(6)=241091


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