Telescoping Problem

1 2015 ! + k = 1 2014 k ( k + 1 ) ! = ? \large \dfrac1{2015!} + \sum_{k=1}^{2014} \dfrac k{(k+1)!} = \, ?

Notation : ! ! denotes the factorial notation. For example, 8 ! = 1 × 2 × 3 × × 8 8! = 1\times2\times3\times\cdots\times8 .


Source : Indonesian Mathematic Olympiad 2014.
1 2014 2 2015

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1 solution

Swapnil Das
Jun 28, 2016

I have divided the question into two parts. One showing the simplified version of the summation, other showing the telescoping significance.


Part I

k = 1 2014 k ( k + 1 ) ! = 1 2 ! + 2 3 ! + 3 4 ! + . . . + 2014 2015 ! \large \displaystyle\sum _{ k=1 }^{ 2014 }{ \dfrac { k }{ \left( k+1 \right) ! } } =\dfrac { 1 }{ 2! } +\dfrac { 2 }{ 3! } +\dfrac { 3 }{ 4! } +...+\dfrac { 2014 }{ 2015! }

Part 2

( 1 2 ! + 2 3 ! + 3 4 ! + . . . + 2014 2015 ! ) + 1 2015 ! = 1 2 ! + 2 3 ! + 3 4 ! + . . + 2014 + 1 2015 ! \huge \left( \frac { 1 }{ 2! } +\frac { 2 }{ 3! } +\frac { 3 }{ 4! } +...+\frac { 2014 }{ 2015! } \right)+\frac { 1 }{ 2015! } =\frac { 1 }{ 2! } +\frac { 2 }{ 3! } +\frac { 3 }{ 4! } +..+\frac { 2014+1 }{ 2015! }

= 1 2 ! + 2 3 ! + 3 4 ! + . . . + 2013 2014 ! + 1 2014 ! = . . . = 1 + 1 2 ! = 1 \huge =\frac { 1 }{ 2! } +\frac { 2 }{ 3! } +\frac { 3 }{ 4! } +...+\frac { 2013 }{ 2014! } +\frac { 1 }{ 2014! } =...=\frac { 1 +1}{ 2! } =1

Note : The final answer follows from the fact that for each k k , we get an expression like k ( k + 1 ) ! + 1 ( k + 1 ) ! = 1 k ! \dfrac{k}{(k+1)!}+ \dfrac {1}{(k+1)!} = \dfrac {1}{k!} which triggers a chain reaction and forms a telescoping series giving 1 1 as the answer.

I don't understand, what is part 1 and 2

Racchit Jain - 4 years, 11 months ago

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I also don't think that your solution is complete. Am I right?

Racchit Jain - 4 years, 11 months ago

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@Racchit Jain I don't see anything wrong with the solution.

Mehul Arora - 4 years, 11 months ago

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@Mehul Arora Now it's perfectly fine

Racchit Jain - 4 years, 11 months ago

@Mehul Arora I think what happened was that I was viewing it on my phone and some part of the solution got cut the mobile website is not working properly on my phone

Racchit Jain - 4 years, 11 months ago

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@Racchit Jain That's okay, no issue. P.S. How much are you scoring in CTT?

Mehul Arora - 4 years, 11 months ago

No earlier it was incomplete

Racchit Jain - 4 years, 11 months ago

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