Bài 1: Tính nhanh: a) 5/7 + 7/13 + 19/13 + 6/5 + 9/7 + 9/5 b) 1/11 + 2/11 + 3/11 + 4/11 + 5/11 + 6/11 + 7/11 + 8/11 + 9/11 + 10/11 c) 1/21 + 2/21 + 3/21 + 4/21 + 5/21 + ....+ 17/21 + 18/21 + 19/21 + 20/21 Bài 2 : Tính nhanh: a) 34/31 - 19/28 - 3/31 b) 18/13 + 55/46 - 5/13 c) 7/3 +11/5 - 4/3 d) ( 27/25 - 4/9 ) - ( 2/25 - 5/9 ) Bài 3: Tính nhanh: a) 4/7 x 5/6 + 3/7 x 5/6 b) 5/9 x 1/4 + 4/9 x 3/12 c) 7/9 x 8/5 - 7/9 x 3/5 d) 2006/2005 x 3/4 - 3/4 x 1/ 2005 Bài 4: Tính bằng cách thuận tiện nhất: 1/2 x 2/3 x 3/4 x 4/5x 5/6 1/10x2/9 x 3/8x 4/7 x 5/6 x 6/5 x 7/4 x 8 giúp mik nha bạn nào mik cũng cho 5 sao

Bài 1:

`a) 5/7 + 7/13 + 19/13 + 6/5 + 9/7 + 9/5`

`= (5/7 + 9/7) + ( 7/13 + 19/13) + ( 6/5 + 9/5)`

`=  2 + 2 + 3`

`= 7`

`b) (1/11 + 10/11) + (2/11 + 9/11) + (3/11 + 8/11) + (4/11 + 7/11) + (5/11 + 6/11)`

`= 11/11 + 11/11 + 11/11 + 11/11 + 11/11`

`= 1 + 1 + 1 + 1 + 1`

`= 5`

`c)` Dãy số có số số hạng là:

`(20/21 - 1/21) : 1/21 + 1 = 20`

Tổng dãy trên là:

`(1/21 + 20/21) × 20 : 2 = 10`

Bài `2`: 

`a)  34/31 - 3/31 - 19/28`

`1 - 19/28 = 9/28`

`b) 18/13 - 5/13 + 55/46`

`= 1 + 55/46`

`= 101/46`

`c) 7/3 - 4/3 + 11/5`

`= 1 + 11/5`

`= 16/5`

`d) ( 27/25 - 2/25 ) - ( 5/9 + 4/9 )`

`= 1 - 1`

`= 0`

Bài `3`:

`a) 4/7 × 5/6 + 3/7 × 5/6`

`= 5/6 × (4/7 + 3/7)`

`= 5/6 × 1`

`= 5/6`

`b) 5/9 × 1/4 + 4/9 + 3/12`

`= 5/9 × 1/4 + 4/9 + 1/4`

`= 1/4 × (5/9 + 1) + 4/9`

`= 1/4 × 14/9 + 4/9`

`= 7/18 + 8/18`

`= 5/6`

`c) 7/9 × 8/5 - 7/9 × 3/5`

`= 7/9 × (8/5 - 3/5)`

`= 7/9 × 1`

`= 7/9`

`d) 2006/2005 × 3/4 - 3/4 × 1/2005`

`= 3/4 × (2006/2005 - 1/2005)`

`= 3/4 × 1`

`= 3/4`

Bài `4`:

`a) 1/2 × 2/3 × 3/4 × 4/5 × 5/6`
`= (1 × 2 × 3 × 4 × 5)/(2 × 3 × 4 × 5 × 6)`
`= 1/6`

`b) 1/10 × 2/9 × 3/8 × 4/7 × 5/6 × 6/5 × 7/4 × 8/3 × 9/2`
`= 1/10 × (2/9 × 9/2) × (3/8 × 8/3) × (4/7 × 7/4) × (5/6 × 6/5)`
`= 1/10 × 1 × 1 × 1 × 1`
`= 1/10`

`o Duong`