Đáp án:

 

Giải thích các bước giải:

 `a)`

` 1/2 + 3/4 - 9/11 + 2/3`

`= ( 1/2 - 9/11 ) + ( 3/4 + 2/3 ) `

`= ( 11/22 - 18/22 ) + ( 9/12 + 8/12 ) `

`= - 7/22 + 17/12 `

`= - 42/132 + 187/132`

`= 145/132`

``

`b)`

` 1/7 - 2/4 + 5/6 - 7/8`

`= ( - 2/4 - 7/8 ) + ( 1/7 + 5/6 )`

`= ( - 4/8 - 7/8 ) + ( 6/42 + 35/42 )`

`= -11/8 + 41/42 `

`= -231/168 + 164/168 `

`= -67/168`