Too much to solve!

Calculus Level 4

Moderator's edit :

If the value of m = 1 26 [ x = 0 n = 1 m + 1 1 x + n ] 1 \displaystyle \sum_{m=1}^{26} \left [\sum_{x=0}^\infty \prod_{n=1}^{m+1} \dfrac1{x+n} \right ]^{-1} can be expressed as ω ! 1 \omega! - 1 , find ω 3 \sqrt[3]{\omega} .


I f x = 0 n = 1 2 1 ( x + n ) = A x = 0 n = 1 3 1 ( x + n ) = B . . . . . . . . . . . . . . x = 0 n = 1 27 1 ( x + n ) = Z t h e n 1 A + 1 B . . . . . . . . . . + 1 Z c a n b e e x p r e s s e d a s ϖ ! 1 f i n d ϖ 3 If\\ \\ \sum _{ x=0 }^{ \infty }{ } \prod _{ n=1 }^{ 2 }{ \frac { 1 }{ (x+n) } } =\quad A\\ \sum _{ x=0 }^{ \infty }{ } \prod _{ n=1 }^{ 3 }{ \frac { 1 }{ (x+n) } } =\quad B\\ ..............\\ \sum _{ x=0 }^{ \infty }{ } \prod _{ n=1 }^{ 27 }{ \frac { 1 }{ (x+n) } } =\quad Z\\ \quad \\ then\quad \frac { 1 }{ A } \quad +\quad \frac { 1 }{ B } \quad ..........\quad +\quad \frac { 1 }{ Z\quad } \quad can\quad be\quad expressed\quad as\quad \varpi !-1\\ \\ find\quad \sqrt [ 3 ]{ \varpi } \\ \\ \\ \\ \\ \\ \\ \\ \\ \\


The answer is 3.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Rishabh Jain
Feb 20, 2016

L e t C = x = 0 1 n = 1 m + 1 ( x + n ) Let~\Large \mathfrak{C}=\displaystyle \sum_{x=0}^{\infty} \dfrac{1}{\displaystyle \prod_{n=1}^{m+1} (x+n)} = 1 m x = 0 ( 1 n = 1 m ( n + x ) 1 n = 2 m + 1 ( n + x ) ) \large =\frac{1}{m}\cdot\sum_{x=0}^{\infty}(\frac{1}{\displaystyle\prod_{n=1}^{m}(n+x)}-\frac{1}{\displaystyle\prod_{n=2}^{m+1}(n+x)})

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

= 1 m ( 1 n = 1 m ( n ) ) \large=\frac{1}{m}(\frac{1}{\displaystyle\prod_{n=1}^{m}(n)})

= 1 m × m ! \Large\color{#3D99F6}{=\dfrac{1}{m\times m!}} m = 1 26 [ x = 0 n = 1 m + 1 1 x + n ] 1 \large \therefore \displaystyle \sum_{m=1}^{26} \left [\sum_{x=0}^\infty \prod_{n=1}^{m+1} \dfrac1{x+n} \right ]^{-1} = m = 1 26 m × m ! = \displaystyle \sum_{m=1}^{26}m\times m! = m = 1 26 ( ( m + 1 ) ! m ! ) \large = \displaystyle \sum_{m=1}^{26} ((m+1)!- m! ) = 27 ! 1 \Large =27!-1 ω 3 = 27 3 = 3 \huge \therefore \sqrt[3]{\omega}= \sqrt[3]{27}=\boxed{\color{#007fff}{3}}

S = m = 1 26 [ x = 0 n = 1 m + 1 1 x + n ] 1 S = \displaystyle \sum_{m=1}^{26} \displaystyle \left[\sum_{x=0}^{\infty} \prod_{n=1}^{m+1} \dfrac{1}{x+n} \right]^{-1}
Consider
[ x = 0 n = 1 m + 1 1 x + n ] = x = 0 1 ( x + 1 ) ( x + 2 ) ( x + 3 ) ( x + m + 1 ) = 1 m × [ 1 ( x + 1 ) ( x + 2 ) ( x + 3 ) ( x + m ) 1 ( x + 2 ) ( x + 3 ) ( x + 4 ) ( x + m + 1 ) ] \displaystyle \left[ \sum_{x=0}^{\infty} \prod_{n=1}^{m+1} \dfrac{1}{x+n} \right] = \displaystyle\sum_{x=0}^{\infty} \dfrac{1}{(x+1)(x+2)(x+3)\ldots(x+m+1)} = \dfrac{1}{m} \times \left[\dfrac{1}{(x+1)(x+2)(x+3)\ldots(x+m)} - \dfrac{1}{(x+2)(x+3)(x+4)\ldots(x+m+1)} \right]
This forms a telescopic series, i.e a series where successive terms cancel out each other and we are left with,


[ x = 0 n = 1 m + 1 1 x + n ] = 1 m × [ 1 m ! lim x 1 ( x + 2 ) ( x + 3 ) ( x + 4 ) ( x + m + 1 ) ] = 1 m m ! \displaystyle \left[ \sum_{x=0}^{\infty} \prod_{n=1}^{m+1} \dfrac{1}{x+n} \right ] = \dfrac{1}{m} \times \left[\dfrac{1}{m!} - \displaystyle \lim_{x \rightarrow \infty} \dfrac{1}{(x+2)(x+3)(x+4)\ldots(x+m+1)}\right] = \dfrac{1}{m\cdot m!}
S = m = 1 26 [ 1 m m ! ] 1 = m = 1 26 m m ! = m = 1 26 ( m + 1 1 ) m ! = m = 1 26 ( m + 1 ) ! m ! S = \displaystyle \sum_{m=1}^{26} \left[\dfrac{1}{m \cdot m!}\right]^{-1} = \displaystyle \sum_{m=1}^{26}m\cdot m! = \sum_{m=1}^{26} (m+1-1)\cdot m! = \sum_{m=1}^{26} (m+1)! - m!
This forms another telecsopic series where the terms cancel out as follows,

S = 2 ! 1 ! + 3 ! 2 ! + 27 ! 26 ! = 27 ! 1 \therefore S = 2! - 1! + 3! - 2! + \ldots 27! - 26! = 27! - 1
S = 27 ! 1 \therefore S = 27! -1
Comparing we get ω = 27 \omega = 27
ω 3 = 27 3 = 3 \sqrt[3]{\omega} = \sqrt[3]{27} = 3

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...