n = 1 ∑ 2 4 n 5 + 1 0 n 4 + 3 5 n 3 + 5 0 n 2 + 2 4 n 7 n 3 + 4 1 n 2 + 8 8 n + 7 2 If the value of the above expression is in the form of B A , where A , B are positive coprime integers then find the value of A + B .
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Let the sum be S , then we have:
S = n = 1 ∑ 2 4 n 5 + 1 0 n 4 + 3 5 n 3 + 5 0 n 2 + 2 4 n 7 n 3 + 4 1 n 2 + 8 8 n + 7 2 = n = 1 ∑ 2 4 n ( n + 1 ) ( n + 2 ) ( n + 3 ) ( n + 4 ) 3 ( n 3 + 9 n 2 + 2 6 n + 2 4 ) + 4 n ( n 2 + 3 n + 2 ) + 2 n ( n + 1 ) = n = 1 ∑ 2 4 n ( n + 1 ) ( n + 2 ) ( n + 3 ) ( n + 4 ) 3 ( n + 2 ) ( n + 3 ) ( n + 4 ) + 4 n ( n + 1 ) ( n + 2 ) + 2 n ( n + 1 ) = n = 1 ∑ 2 4 ( n ( n + 1 ) 3 + ( n + 3 ) ( n + 4 ) 4 + ( n + 2 ) ( n + 3 ) ( n + 4 ) 2 ) = n = 1 ∑ 2 4 ( 3 [ n 1 − n + 1 1 ] + 4 [ n + 3 1 − n + 4 1 ] + [ n + 2 1 − n + 3 2 + n + 4 1 ] ) = 3 n = 1 ∑ 2 4 n 1 − 3 n = 1 ∑ 2 4 n + 1 1 + n = 1 ∑ 2 4 n + 2 1 + 2 n = 1 ∑ 2 4 n + 3 1 − 3 n = 1 ∑ 2 4 n + 4 1 = 3 n = 1 ∑ 2 4 n 1 − 3 n = 2 ∑ 2 5 n 1 + n = 3 ∑ 2 6 n 1 + 2 n = 4 ∑ 2 7 n 1 − 3 n = 5 ∑ 2 8 n 1 = 1 3 − 2 5 3 + 3 1 + 4 3 − 2 7 1 − 2 8 3 = 2 5 7 4 + 2 7 8 + 1 4 9 = 9 4 5 0 2 7 2 1 6 + 2 8 0 0 + 6 0 7 5 = 9 4 5 0 3 6 0 9 1
⇒ A + B = 3 6 0 9 1 + 9 4 5 0 = 4 5 5 4 1
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In the true spirit of bashing, we hope to find the partial fraction form of the expression.
By inspection (rational root theorem helps too), observe that
n 5 + 1 0 n 4 + 3 5 n 3 + 5 0 n 2 + 2 4 n = n ( n + 1 ) ( n + 2 ) ( n + 3 ) ( n + 4 )
Hence let n 5 + 1 0 n 4 + 3 5 n 3 + 5 0 n 2 + 2 4 n 7 n 3 + 4 1 n 2 + 8 8 n + 7 2 = n A + n + 1 B + n + 2 C + n + 3 D + n + 4 E
Multiplying by the original denominator on both sides, 7 n 3 + 4 1 n 2 + 8 8 n + 7 2 = A ( n + 1 ) ( n + 2 ) ( n + 3 ) ( n + 4 ) + B n ( n + 2 ) ( n + 3 ) ( n + 4 ) + C n ( n + 1 ) ( n + 3 ) ( n + 4 ) + D n ( n + 1 ) ( n + 2 ) ( n + 4 ) + E n ( n + 1 ) ( n + 2 ) ( n + 3 )
Now, the easiest way is to set n = 0 , − 1 , − 2 , − 3 , − 4 to get a linear equation in terms of each coefficient A,B,C,D and E respectively.
Note: If this were to be a formal proof, we would have to expand and compare coefficients, since by right n cannot be equal to -4,-3,-2,-1,0 otherwise we would have division by zero. Personally I feel this informal method is much faster.
We then obtain ( A , B , C , D , E ) = ( 3 , − 3 , 1 , 2 , − 3 )
n 5 + 1 0 n 4 + 3 5 n 3 + 5 0 n 2 + 2 4 n 7 n 3 + 4 1 n 2 + 8 8 n + 7 2 = n 3 − n + 1 3 + n + 2 1 + n + 3 2 − n + 4 3
Finally n = 1 ∑ 2 4 n 5 + 1 0 n 4 + 3 5 n 3 + 5 0 n 2 + 2 4 n 7 n 3 + 4 1 n 2 + 8 8 n + 7 2 = n = 1 ∑ 2 4 ( n 3 − n + 1 3 + n + 2 1 − n + 4 1 + n + 3 2 − n + 4 2 ) = 3 n = 1 ∑ 2 4 ( n 1 − n + 1 1 ) + n = 3 ∑ 2 6 ( n 1 − n + 2 1 ) + 2 n = 4 ∑ 2 7 ( n 1 − n + 1 1 ) = 3 ( 1 − 2 5 1 ) + ( 3 1 + 4 1 − 2 7 1 − 2 8 1 ) + 2 ( 4 1 − 2 8 1 ) = 9 4 5 0 3 6 0 9 1 ⇒ A + B = 4 5 5 4 1