giải hộ e ạ, giải dễ hiểu dc khog ạ

`1.`

`(3/7 - 3/17 + 3/37) / (5/7 - 5/17 + 5/37)`

`= (3 * 1/7 - 3 * 1/17 + 3 * 1/37)/(5 * 1/7 - 5 * 1/17 + 5 * 1/37)`

`= [3 * (1/7 - 1/17 + 1/37)] / [5 * (1/7 - 1/17 + 1/37)]`

`= 3/5`

`2.`

`11^2 / (13 * 20) + 11^2 / (20 * 27) + 11^2 / (27 * 34) + ... + 11^2 / (62 * 69)`

`= 121 / (13 * 20) + 121 / (20 * 27) + 121 / (27 * 34) + ... + 121 / (62 * 69)`

`= 121 * (1 / (13 * 20) + 1 / (20 * 27) + 1 / (27 * 34) + ... + 1 / (62 * 69))`

`= 121 / 7 * (7 / (13 * 20) + 7 / (20 * 27) + 7 / (27 * 34) + ... + 7 / (62 * 69))`

`= 121 / 7 * (1/13 - 1/20 + 1/20 - 1/27 + 1/27 - 1/34 + ... + 1/62 - 1/69)`

`= 121 / 7 * (1/13 - 1/69)`

`= 121 * 8/897`

`= 968/897`

`3.`

`10/56 + 10/140 + 10/260 + ... + 10/1400`

`= 5/28 + 5/70 + 5/130 + ... + 5/700`

`= 5 / (4 * 7) + 5 / (7 * 10) + 5 / (10 * 13) + ... + 5 / (25 * 28)`

`= 5 * (1 / (4 * 7) + 1 / (7 * 10) + 1 / (10 * 13) + ... + 1 / (25 * 28))`

`= 5 / 3 * (3 / (4 * 7) + 3 / (7 * 10) + 3 / (10 * 13) + ... + 3 / (25 * 28))`

`= 5 / 3 * (1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + ... + 1/25 - 1/28)`

`= 5 / 3 * (1/4 - 1/28)`

`= 5 / 3 * 3/14`

`= 5/14`