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Algebra Level 5

5 3 4 ( 3 ! + 4 ! + 5 ! ) 6 3 5 ( 4 ! + 5 ! + 6 ! ) + + 201 7 3 2016 ( 2015 ! + 2016 ! + 2017 ! ) \dfrac{5^3}{4(3!+4!+5!)}-\dfrac{6^3}{5(4!+5!+6!)}+\cdots+\dfrac{2017^3}{2016(2015!+2016!+2017!)}

If the sum of above series can be written as 1 a ! + 1 b ! + e c + d \dfrac{1}{a!}+\dfrac{1}{b!}+e^c+d for integers a , b , c , d a, b, c, d , then submit a + b + c + d a+b+c+d .


Inspiration

2019 None of the others 2021 2017 2018 2022 2016 2020

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1 solution

Rishabh Jain
Jan 4, 2017

The sum ( S ) (\mathfrak{S}) is: k = 4 2016 [ ( 1 ) k ( k + 1 ) 3 k ( ( k 1 ) ! + k ! + ( k + 1 ) ! ) ] \sum_{k=4}^{2016}\left[(-1)^k\dfrac{ (k+1)^3}{k((k-1)!+k!+(k+1)!)}\right] The denominator is: k ( ( k 1 ) ! + k ! + ( k + 1 ) ! ) = k ( k 1 ) ! ( 1 + k + k ( k + 1 ) ) = k ! ( k 2 + 2 k + 1 ) = k ! ( k + 1 ) 2 \small{ k((k-1)!+k!+(k+1)!)=k(k-1)!(1+k+k(k+1))=k!(k^2+2k+1)=k!(k+1)^2}

S = k = 4 2016 [ ( 1 ) k k + 1 k ! ] = k = 4 2016 ( 1 ) k [ 1 ( k 1 ) ! + 1 k ! ] \therefore \mathfrak{S}= \sum_{k=4}^{2016}\left[(-1)^k\dfrac{k+1}{k!}\right]=\sum_{k=4}^{2016}(-1)^k\left[\dfrac{1}{(k-1)!}+\dfrac{1}{k!}\right]

Thus expanding, we get:

S = 1 3 ! + 1 4 ! 1 4 ! 1 5 ! + 1 5 ! + 1 6 ! + + 1 2015 ! + 1 2016 ! \mathfrak S=\dfrac{1}{3!}+\cancel{\dfrac{1}{4!}}\color{#3D99F6}{-\cancel{\dfrac{1}{4!}}-\cancel{\dfrac{1}{5!}}}\color{#333333}{+\cancel{\dfrac{1}{5!}}+\cancel{\dfrac{1}{6!}}}+\cdots\color{#3D99F6}{+\cancel{\dfrac{1}{2015!}}+\dfrac{1}{2016!}}

= 1 3 ! + 1 2016 ! \large =\dfrac{1}{3!}+\dfrac{1}{2016!}

a = 3 , b = 2016 , c = 0 , d = 1 a + b + c + d = 2018 \therefore a=3,b=2016,c=0,d=-1\implies a+b+c+d=\boxed{\color{cyan}{2018}}

Shouldn't this be in algebra ,rather than calculus?

Sumanth R Hegde - 4 years, 5 months ago

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Yeah... Manipulations are algebraic but the essence of series (convergence/divergence) lies in calculus. Hopefully here it isn't the case, otherwise if it were divergent calculus would have come into play... :-)

Rishabh Jain - 4 years, 5 months ago

Given that this is a finite partial fraction summation, I have updated the topic to Algebra.

There is no concern about convergence / divergence here.

Calvin Lin Staff - 4 years, 5 months ago

i never realised c could be 0!!!

thought i made a mistake and saw solution :(

Rohith M.Athreya - 4 years, 5 months ago

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Ya.. That's the reason why I introduced c to make the problem solver rethink and check steps.. :-)

Rishabh Jain - 4 years, 5 months ago

This problem seems like one of my problem.. But this one is more awesome!! @Rishabh Cool

Fidel Simanjuntak - 4 years, 5 months ago

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Yup... Modified form of what you posted... :-)

Rishabh Jain - 4 years, 5 months ago

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Ehehehe, your are more creative than me.. Nice problem!!

Fidel Simanjuntak - 4 years, 5 months ago

Nevertheless I have mentioned it in the question in Inspiration

Rishabh Jain - 4 years, 5 months ago

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