Đáp án:

Giải thích các bước giải:

 F=`2/(4x9) + 2/(9x14) + 2/(14x19) +....+ 1/(99x104)`

f=`(2x5)/(4x9x5) + (2x5)/(9x14x5) + (2x5)/(14x19x5) +...+(1x5)/(99x104x5)`

f=`2/5 x 5/(4x9) + 2/5 x 5/(9x14) + 2/5 x 5/(14x19) +....+1/5 x 5/(99x104)`

f=`2/5 x (5/(4x9) + 5/(9x14) + 5/(14x19) +...+5/(94x99) )+1/5 x 5/(99x104)`

f=`2/5 x (1/4 - 1/9 + 1/9- 1/14 + 1/14 -1/19 +...+1/94 - 1/99) +  1 x 1/(99x104)`

f=`2/5x(1/4 - 1/9+1/9-1/4+1/14-1/19+...+1/94-1/99+1/99-1/104)`

f=`2/5  x (1/4-1/104)`

f=`2/5 x 25/104`

f=`5/52`

vậy__