tìm n biết: 1/1 + 1/1+2 + 1/1+2+3 +...+ 1/1+2+3+...+n = 200/101

`1/1 +1/(1+2) + 1/(1+2+3) +....+1/(1+2+3+....n) = 200/101`

`1/((1.2)/2) +1/((2.3)/2) +1/((3.4)/2) +.....1/((n.(n+1))/2 = 200/101`

` 2 .(1/(1.2) +1/(2.3) +1/(3.4) +....+1/(n.(n+1)) = 200/101`

` 2 . (1 -1/2 +1/2 -1/3 +1/3 -....-1/(n+1)) = 200/101`

` 2 .(1 -1/(n+1)) = 200/101`

`1-1/(n+1) = 200/101 : 2`

`1-1/(n+1) = 100/101`

`1/(n+1) = 1 - 100/101`

`1/(n+1) = 1/101`

`n +1 = 101`

`n = 100`

   Vậy `n = 100`

`n +1 = 101/100`

`n = 1/100`

   Vậy `n = 1/100`