Notation : denotes the factorial notation. For example, .
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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 ∑ 2 0 1 4 ( k + 1 ) ! k = 2 ! 1 + 3 ! 2 + 4 ! 3 + . . . + 2 0 1 5 ! 2 0 1 4
Part 2
( 2 ! 1 + 3 ! 2 + 4 ! 3 + . . . + 2 0 1 5 ! 2 0 1 4 ) + 2 0 1 5 ! 1 = 2 ! 1 + 3 ! 2 + 4 ! 3 + . . + 2 0 1 5 ! 2 0 1 4 + 1
= 2 ! 1 + 3 ! 2 + 4 ! 3 + . . . + 2 0 1 4 ! 2 0 1 3 + 2 0 1 4 ! 1 = . . . = 2 ! 1 + 1 = 1
Note : The final answer follows from the fact that for each k , we get an expression like ( k + 1 ) ! k + ( k + 1 ) ! 1 = k ! 1 which triggers a chain reaction and forms a telescoping series giving 1 as the answer.