Hộ ạ nhanh xíu nhó :)

Câu `3:`

`a) `

`21/25 × 2/5 + 21/25 × 3/5`

`= 21/25 × (2/5 + 3/5) `

`= 21/25 × 5/5 `

`= 21/25`

`b)`

`5/2 × 3/4 − 3/14 : 6/7 `

`= 15/8 − 3/14 × 7/6 `

`= 15/8 − 21/84 `

`= 15/8 − 1/4`

`= 15/8 − 2/8`

`= 13/8`

`c) `

`3/10 × 7/4 − 3/10 × 3/4 `

`= 3/10 × (7/4 − 3/4) `

`= 3/10 × 4/4`

`= 3/10 `

`d)`

`7/12 + 6/9 + 3/8 + 5/12 + 1/3 + 5/8 `

`= (7/12 + 5/12) + (6/9 + 1/3) + (3/8 + 5/8)`

`= 12/12 + (2/3 + 1/3) + 8/8 `

`= 1 + 1 + 1 `

`= 3`

Câu `4:`

`b)`

`4/5 + 2/7 + 5/9 + 15/21 + 1/5 + 4/9`

`= (4/5 + 1/5) + (2/7 + 15/21) + (5/9 + 4/9)`

`= 5/5 + (2/7 + 5/7) + 9/9`

`= 1 + 1 + 1`

`= 3`

`c)`

`4/7 : 5/2 + 4/7 : 5/3 + 3/7`

`= 4/7 × 2/5 + 4/7 × 3/5 + 3/7`

`= 4/7 × (2/5 + 3/5) + 3/7`

`= 4/7 × 1 + 3/7`

`= 7/7`

`= 1`

`d)`

`3/7 × 24/15 − 3/7 × 14/15`

`= 3/7 × (24/15 − 14/15)`

`= 3/7 × 10/15`

`= 3/7 × 2/3`

`= 2/7`