4 ( 3 ! + 4 ! + 5 ! ) 5 3 − 5 ( 4 ! + 5 ! + 6 ! ) 6 3 + ⋯ + 2 0 1 6 ( 2 0 1 5 ! + 2 0 1 6 ! + 2 0 1 7 ! ) 2 0 1 7 3
If the sum of above series can be written as a ! 1 + b ! 1 + e c + d for integers a , b , c , d , then submit a + b + c + d .
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Shouldn't this be in algebra ,rather than calculus?
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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... :-)
Given that this is a finite partial fraction summation, I have updated the topic to Algebra.
There is no concern about convergence / divergence here.
i never realised c could be 0!!!
thought i made a mistake and saw solution :(
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Ya.. That's the reason why I introduced c to make the problem solver rethink and check steps.. :-)
This problem seems like one of my problem.. But this one is more awesome!! @Rishabh Cool
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Yup... Modified form of what you posted... :-)
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Ehehehe, your are more creative than me.. Nice problem!!
Nevertheless I have mentioned it in the question in Inspiration
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The sum ( S ) is: k = 4 ∑ 2 0 1 6 [ ( − 1 ) k k ( ( k − 1 ) ! + k ! + ( k + 1 ) ! ) ( k + 1 ) 3 ] 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
∴ S = k = 4 ∑ 2 0 1 6 [ ( − 1 ) k k ! k + 1 ] = k = 4 ∑ 2 0 1 6 ( − 1 ) k [ ( k − 1 ) ! 1 + k ! 1 ]
Thus expanding, we get:
S = 3 ! 1 + 4 ! 1 − 4 ! 1 − 5 ! 1 + 5 ! 1 + 6 ! 1 + ⋯ + 2 0 1 5 ! 1 + 2 0 1 6 ! 1
= 3 ! 1 + 2 0 1 6 ! 1
∴ a = 3 , b = 2 0 1 6 , c = 0 , d = − 1 ⟹ a + b + c + d = 2 0 1 8