If x 1 , x 2 , . . . . . , x 7 1 satisfy
j = 1 ∑ 7 1 i + j x j = 2 i + 1 4 for i = 1 , 2 , . . . , 7 1 .
Suppose ∑ j = 1 7 1 2 j + 1 x j = q p , where p and q are coprime integers. Find p + q .
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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
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The given expression in not true for fractions, its only true for i =1,2,....71
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I am giving a solution by replacing 7 1 by n
First define a rational function R ( t ) = ∑ j = 1 n t + j x j
R ( t ) = Q ( t ) P ( t ) for some polynomials P ( t ) and Q ( t ) where d e g P < n
Q ( t ) = ( t + 1 ) ( t + 2 ) . . . . ( t + n )
Define G ( t ) = P ( t ) ( 2 t + 1 ) − 4 Q ( t )
G ( t ) = 0 for t = 1 , . . . . . , n and hence G ( t ) = c ( t − 1 ) . . . . ( t − n ) where c is a constant.
By putting t = − 2 1 show that c = 4 ( − 1 ) n − 1 2 n + 1 1
Now its easy to show ∑ j = 1 n 2 j + 1 x j = 2 1 R ( 2 1 ) = 1 − ( 2 n + 1 ) 2 1
Now, ( 2 n + 1 ) 2 and ( 2 n + 1 ) 2 − 1 are coprime integers.
Hence,by putting n = 7 1 ,we get p + q = 1 4 3 2 + 1 4 3 2 − 1 = 4 0 8 9 7