A respect for polynomials

Algebra Level 5

Let f ( x ) f(x) and g ( x ) g(x) be monic cubic polynomials, such that

  • f ( x ) + g ( x ) = p ( x ) f(x)+g(x) = p (x) ,

  • p ( 1 ) = 13 p(1) = 13 ,

  • p ( 3 ) = 97 p(3) = 97 ,

  • p ( 5 ) = 349 p(5) = 349 ,

  • f ( 7 ) = 491 f(7) = 491 ,

  • g ( 19 ) = 6938 g(19) = 6938 , and

  • g ( x ) g(x) is a depressed cubic polynomial.

Evaluate f ( 30 ) + g ( 27 ) f(30) + g (27) .


The answer is 49495.

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1 solution

Alexander Becker
Apr 5, 2018

f ( x ) f(x) and g ( x ) g(x) being monic cubic polynomials, g ( x ) g(x) depressed:

f ( x ) = x 3 + a x 2 + c x + d f(x)=x^3+ax^2+cx+d ,

g ( x ) = x 3 + e x + f g(x)=x^3+ex+f ,

p ( x ) = f ( x ) + g ( x ) = 2 x 3 + a x 2 + ( c + e ) x + d + f p(x)=f(x)+g(x)=2x^3+ax^2+(c+e)x+d+f .

Plugging in the given values...

p ( 1 ) = 13 2 + a + c + d + e + f = 13 a + c + d + e + f = 11 p(1)=13 \Rightarrow 2+a+c+d+e+f=13 \Rightarrow a+c+d+e+f=11

p ( 3 ) = 97 54 + 9 a + 3 c + d + 3 e + f = 97 9 a + 3 c + d + 3 e + f = 43 p(3)=97 \Rightarrow 54+9a+3c+d+3e+f=97 \Rightarrow 9a+3c+d+3e+f=43

\ldots

...results in a linear system of equations, whose solution is a = 3 , c = 0 , d = 1 , e = 4 , f = 3 a=3, c=0, d=1, e=4, f=3 , so f ( x ) = x 3 + 3 x 2 + 1 f(x)=x^3+3x^2+1 and g ( x ) = x 3 + 4 x + 3 g(x)=x^3+4x+3 , and f ( 30 ) + g ( 27 ) = 49495 f(30)+g(27)=\boxed{49495} .

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