Tính nhanh: A = 2/3 + 4/21 + 5/84 + 6/216 + 9/486 + 12/1053

`A = 2/3 + 4/21 + 5/84 + 6/216 + 9/846 + 12/1053`

`A = 2/(1 xx 3) + 4/(3 xx 7) + 5/(7 xx 12) + 6/(12 xx 18) + 9/(18 xx 27) + 12/(27 xx 39)`

`A = (3 - 1)/(1 xx 3) + (7 - 3)/(3 xx 7) + (12 - 7)/(7 xx 12) + (18 - 12)/(12 xx 18) + (27 - 18)/(18 xx 27) + (39 - 27)/(27 xx 39)`

`A = 3/(1 xx 3) - 1/(1 xx 3) + 7/(3 xx 7) - 3/(3 xx 7) + 12/(7 xx 12) - 7/(7 xx 12) + 18/(12 xx 18) - 12/(18 xx 12) + 27/(18 xx 27) - 18/(18 xx 27) + 39/(39 xx 27) - 27/(27 xx 39)`

`A = 1 - 1/3 + 1/3 - 1/7 + 1/7 - 1/12 - 1/12 + 1/18 - 1/18 - 1/27 + 1/27 - 1/39`

`A = 1 - 1/39`

`A = 38/39`