20172 2 3 2 2016 2+ + + +2 1 3 1 4 1 2016 1= + + + + + 1 1 1 2 3 4

1 . . .... .

F = + + + +

Ta có: 1 2.3 1 3.4 1 2016.2017

2 2 3 2 2016 2

+ + + +

2 1 3 1 4 1 2016 1

= + + + + + 1 1 1 2 3 4 ... 2016

= + + + + +