Bài 2 : Tính nhanh :

`5/13 + (-5)/7 + (-20)/41 + 8/13 + (-21)/41`

`= ( 5/13 + 8/13 ) + ( -20/41 - 21/41 ) - 5/7`

`= 1 + (-1) - 5/7`

`= 0 - 5/7`

`= -5/7`

`-----`

`1/15 + 1/35 + 1/63 + 1/99 + 1/143`

`= 1/(3 . 5 ) + 1/( 5 . 7 ) + 1/(7 . 9 ) + 1/(9.11)`

`= 1/2 ( 2/(3 . 5 ) + 2/( 5 . 7 ) + 2/( 7 . 9 ) + 2/(9.11) )`

`= 1/2 . ( 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 )`

`= 1/2 . ( 1/3 - 1/11 )`

`= 1/2 . 8/33`

`= 4/33`

`-----`

`1/5 . 4/7 + 3/7 . 1/5 - 1/5`

`= 1/5 . ( 4/7 + 3/7 ) - 1/5`

`= 1/5 . 1 - 1/5`

 `= 0`

`-----`

`-3/7 . 15/13 - 3/7 . 11/13 - 3/7`

`= -3/7 . ( 15/13 + 11/13 + 1 )`

`= -3/7 . 3`

`= -7`

`#tm`