Not ordinary sum-2

Calculus Level 5

n = 1 n 4 4 n n ! = c e a π d 1 b \large \sum _{ n=1 }^{ \infty }{ \dfrac { { n }^{ 4} }{ { 4 }^{ n }\cdot n! } } =\dfrac { c~\sqrt [ a ]{ e } ~\pi^{d-1}}{ b }

If the equation above holds true for positive integers a , b , c , d a,b,c,d , with b , c b, c coprime, find a + b + c + d a+b+c+d .

Clarification : e 2.71828 e \approx 2.71828 denotes the Euler's number .


Try part-1 here .


The answer is 462.

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1 solution

Rishabh Jain
Jul 6, 2016

Relevant wiki: Taylor Series Manipulation

Write the sum as:

n = 1 ( n ( n 1 ) ( n 2 ) ( n 3 ) + ( 6 n ( n 1 ) ( n 2 ) ) + ( 7 n ( n 1 ) ) + n 4 n n ! \displaystyle\large \sum _{ n=1 }^{ \infty }{ \dfrac { (n(n-1)(n-2)(n-3)+(6n(n-1)(n-2))+(7n(n-1))+n}{ { 4 }^{ n }\cdot n! } }

= n = 1 n ( n 1 ) ( n 2 ) ( n 3 ) 4 n n ! + 6 n = 1 n ( n 1 ) ( n 2 ) 4 n n ! + 7 n = 1 n ( n 1 ) 4 n n ! + n = 1 n 4 n n ! =\displaystyle\sum _{ n=1 }^{ \infty }{ \dfrac { n(n-1)(n-2)(n-3)}{ { 4 }^{ n }\cdot n! } }+6\displaystyle\sum _{ n=1 }^{ \infty }{ \dfrac { n(n-1)(n-2)}{ { 4 }^{ n }\cdot n! } }+7\displaystyle\sum _{ n=1 }^{ \infty }{ \dfrac { n(n-1)}{ { 4}^{ n }\cdot n! } }+\displaystyle\sum _{ n=1 }^{ \infty }{ \dfrac { n}{ {4}^{ n }\cdot n! } }

= n = 4 1 4 n ( n 4 ) ! + 6 n = 3 1 4 n ( n 3 ) ! + 7 n = 2 1 4 n ( n 2 ) ! + n = 1 1 4 n ( n 1 ) ! =\displaystyle\sum _{ n=4 }^{ \infty }{ \dfrac {1}{ { 4 }^{ n }\cdot (n-4)! } }+6\displaystyle\sum _{ n=3 }^{ \infty }{ \dfrac { 1}{ { 4 }^{ n }\cdot ( n-3)! } }+7\displaystyle\sum _{ n=2 }^{ \infty }{ \dfrac { 1}{ { 4 }^{ n }\cdot (n-2)! } }+\displaystyle\sum _{ n=1 }^{ \infty }{ \dfrac { 1}{ { 4 }^{ n }\cdot (n-1)! } }

= 1 4 4 n = 4 ( 1 4 ) n 4 ( n 4 ) ! + 6 4 3 n = 3 ( 1 4 ) n 3 ( n 3 ) ! + 7 4 2 n = 2 ( 1 4 ) n 2 ( n 2 ) ! + 1 4 n = 1 ( 1 4 ) n 1 ( n 1 ) ! =\dfrac{1}{4^4}\displaystyle\sum _{ n=4 }^{ \infty }{ \dfrac { \left(\frac 14\right)^{n-4}}{ (n-4)! } }+\dfrac 6{4^3}\displaystyle\sum _{ n=3 }^{ \infty }{ \dfrac { \left(\frac 14\right)^{n-3}}{ ( n-3)! } }+\dfrac 7{4^2}\displaystyle\sum _{ n=2 }^{ \infty }{ \dfrac { \left(\frac 14\right)^{n-2}}{ ( n-2)! } }+\dfrac 14\displaystyle\sum _{ n=1 }^{ \infty }{ \dfrac { \left(\frac 14\right)^{n-1}}{ (n-1)! } }

= 1 4 4 e 1 / 4 + 6 4 3 e 1 / 4 + 7 4 2 e 1 / 4 + 1 4 e 1 / 4 ( e x = r = 0 x r r ! ) =\dfrac 1{4^4}e^{1/4}+\dfrac 6{4^3}e^{1/4}+\dfrac 7{4^2}e^{1/4}+\dfrac 14e^{1/4}~~\left(\small{\because \color{#3D99F6}{e^x=\displaystyle\sum_{r=0}^{\infty}\dfrac{x^r}{r!}}}\right)

= 201 e 1 / 4 π 0 256 \large =\dfrac{201e^{1/4}\pi^0}{256}

4 + 256 + 201 + 1 = 462 \Large \therefore 4+256+201+1=\boxed{\color{#D61F06}{462}}

Is that a typo in the 4th line ? 1 3 n n ! \quad \dfrac{1}{3^n\cdot n!}

Sabhrant Sachan - 4 years, 11 months ago

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Yup... Thanks ... Corrected...

Rishabh Jain - 4 years, 11 months ago

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