I Thought These Were Partial Fractions

Algebra Level 5

If x 1 , x 2 , . . . . . , x 71 x_{1},x_{2},.....,x_{71} satisfy

j = 1 71 x j i + j = 4 2 i + 1 for i = 1 , 2 , . . . , 71. \sum_{j=1}^{71}\frac{x_{j}}{i+j}=\frac{4}{2i+1} \text{ for } i=1,2,...,71.

Suppose j = 1 71 x j 2 j + 1 = p q , \sum_{j=1}^{71}\frac{x_{j}}{2j+1} = \frac{p}{q}, where p p and q q are coprime integers. Find p + q p+q .


The answer is 40897.

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1 solution

Souryajit Roy
May 7, 2014

I am giving a solution by replacing 71 71 by n n

First define a rational function R ( t ) = j = 1 n x j t + j R(t)=\sum_{j=1}^{n}\frac{x_{j}}{t+j}

R ( t ) = P ( t ) Q ( t ) R(t)=\frac{P(t)}{Q(t)} for some polynomials P ( t ) P(t) and Q ( t ) Q(t) where d e g P < n degP<n

Q ( t ) = ( t + 1 ) ( t + 2 ) . . . . ( t + n ) Q(t)=(t+1)(t+2)....(t+n)

Define G ( t ) = P ( t ) ( 2 t + 1 ) 4 Q ( t ) G(t)=P(t)(2t+1)-4Q(t)

G ( t ) = 0 G(t)=0 for t = 1 , . . . . . , n t=1,.....,n and hence G ( t ) = c ( t 1 ) . . . . ( t n ) G(t)=c(t-1)....(t-n) where c c is a constant.

By putting t = 1 2 t=-\frac{1}{2} show that c = 4 ( 1 ) n 1 1 2 n + 1 c=4(-1)^{n-1}\frac{1}{2n+1}

Now its easy to show j = 1 n x j 2 j + 1 = 1 2 R ( 1 2 ) = 1 1 ( 2 n + 1 ) 2 \sum_{j=1}^{n}\frac{x_{j}}{2j+1}=\frac{1}{2}R(\frac{1}{2})=1-\frac{1}{(2n+1)^{2}}

Now, ( 2 n + 1 ) 2 (2n+1)^{2} and ( 2 n + 1 ) 2 1 (2n+1)^{2}-1 are coprime integers.

Hence,by putting n = 71 n=71 ,we get p + q = 14 3 2 + 14 3 2 1 = 40897 p+q=143^{2} +143^{2}-1=40897

take 2 outside from the denominator (because it's summation) , it will have the form of the given equation and put the value 1/2 . [4/(2i+1)]*1/2

Shrinit Singh - 7 years, 1 month ago

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The given expression in not true for fractions, its only true for i =1,2,....71

Manish Kansal - 7 years ago

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