Sumproduct For Next Year

Calculus Level 5

i = 1 ( n = 1 2017 ( i + n ) ) 1 \large \displaystyle \sum_{i=1}^{\infty} \left ( {\displaystyle \prod_{n=1}^{2017} (i+n)} \right)^{-1}

If the value of above expression is in the form 1 a b ! \dfrac{1}{a \cdot b!} , where a a and b b are positive integers with a < b a<b , find a + b a+b .

Bonus : Generalise it.


The answer is 4033.

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1 solution

Rishabh Jain
Feb 18, 2016

L e t C = i = a b 1 n = p q ( i + n ) Let~\Large \mathfrak{C}=\displaystyle \sum_{i=a}^{b} \dfrac{1}{\displaystyle \prod_{n=p}^{q} (i+n)} = 1 q p i = a b ( 1 n = p q 1 ( n + i ) 1 n = p + 1 q ( n + i ) ) \large =\frac{1}{q-p}\cdot\sum_{i=a}^b(\frac{1}{\displaystyle\prod_{n=p}^{q-1}(n+i)}-\frac{1}{\displaystyle\prod_{n=p+1}^{q}(n+i)})

(A TELESCOPIC SERIES) \large\color{#D61F06}{\mathcal{\text{(A TELESCOPIC SERIES)}}}

= 1 q p ( 1 n = p q 1 ( a + n ) 1 n = p + 1 q ( b + n ) ) \large=\frac{1}{q-p}(\frac{1}{\displaystyle\prod_{n=p}^{q-1}(a+n)}-\frac{1}{\displaystyle\prod_{n=p+1}^{q}(b+n)})

C = 1 q p ( ( a + p 1 ) ! ( a + q 1 ) ! ( b + p ) ! ( b + q ) ! ) \Large\color{#3D99F6}{\boxed{\mathfrak{C}=\color{#333333}{\frac{1}{q-p}(\frac{(a+p-1)!}{(a+q-1)!}-\frac{(b+p)!}{(b+q)!})}}}

For this question a = p = 1 , q = 2017 and b \color{#20A900}{a=p=1, q=2017 \text{ and } b\rightarrow\infty} i = 1 1 n = 1 2017 ( i + n ) \large \therefore~\displaystyle \sum_{i=1}^{\infty} \dfrac{1}{\prod_{n=1}^{2017} (i+n)} = 1 2016 × 2017 ! \large =\dfrac{1}{2016\times 2017!}\\ 2016 + 2017 = 4033 \huge \therefore~2016+2017=\boxed{\color{#007fff}{4033}}


Assumptions: ( p q , ( n + i ) 0 , a < b , p < q ) i , n , a , b , p , q Z + \color{#302B94}{\text{Assumptions:}}\\\small{(p\neq q,(n+i)\neq 0, a< b, p< q)\forall i,n,a,b,p,q\in \mathbb{Z}^+ }

Did it the same way (+1)!

Harsh Khatri - 5 years, 3 months ago

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