Logarithmic Polynomial?

Algebra Level 5

P ( x ) P(x) is a polynomial with degree of 5. Given that P ( 1 ) = 0 P(1)=0 , P ( 3 ) = 1 P(3)=1 , P ( 9 ) = 2 P(9)=2 , P ( 27 ) = 3 P(27) =3 , P ( 81 ) = 4 P(81)=4 and P ( 243 ) = 5 P(243)=5 . What is the coefficient of x x in the polynomial? If the answer can be expressed in the form of a b \frac{a}{b} , where a a and b b are coprime positive integers, find the value of a + b a+b ?


The answer is 283.

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1 solution

Mark Hennings
Feb 23, 2017

Define the quantic polynomials p j ( x ) = 0 i 5 , i j ( x 3 i ) 0 j 5 p_j(x) \; = \; \prod_{{0 \le i \le 5} \,,\, {i \neq j}} (x - 3^i) \hspace{2cm} 0 \le j\le 5 Then p j ( 3 i ) = 0 p_j(3^i) = 0 for any 0 i , j 5 0 \le i,j\le 5 with i j i \neq j , and so the polynomial we want is P ( x ) = j = 1 5 j p j ( 3 j ) p j ( x ) = 1 14289858 x 5 121 4723920 x 4 + 605 255879 x 3 605 8748 x 2 + 121 162 x 85583 125840 \begin{aligned} P(x) & = \sum_{j=1}^5 \frac{j}{p_j(3^j)}p_j(x) \\ & = \tfrac{1}{14289858}x^5 - \tfrac{121}{4723920}x^4 + \tfrac{605}{255879}x^3 - \tfrac{605}{8748}x^2 + \tfrac{121}{162}x - \tfrac{85583}{125840} \end{aligned} making the answer 121 + 162 = 283 121 + 162 = \boxed{283} .

Can the calculation be done manually by hand in a reasonable , elegant way? I had to use matrices and wolfram alpha for this question

Jacob Sony - 1 year, 11 months ago

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