neu theo de cua minh` thi` co cach lam` nhu sau :
(1-1\1+2)(1-1\1+2+3)....(1-1\1+2+3+...+n)= 2\3 . 5\6 . .... . (n+1)n:2 -1 \ (n+1)n:2= 4\6 . 10\12 . .... . (n+1)n -2\ (n+1)n
= (1.4) \ (2.3) . (2.5)\(3.4) . ..... . (n-1)(n+2)\(n+1)n = (1.4).(2.5). ..... .(n-1).(n+2)\(2.3).(3.4) . ..... .n(n+1) =
1.2. ..... .(n-1) . 4.5. .... .(n+2) \ 2.3. ..... .n . 3.4. ...... .(n+1) = n+2 \ n.3 => A\B = n+2 \ 3n : n+2 \ n = 1\3 .