Đáp án+Giải thích các bước giải:
Câu `4`:
`(2019xx2018-2017)/(2019+2017xx2018)`
`=(2019xx2018-2018+1)/(2018+1+2017xx2018)`
`=((2019xx2018-2018)+1)/((2018xx2017+2018)+1)`
`=(2018xx(2019-1)+1)/(2018xx(2017+1)+1)`
`=(2018xx2018+1)/(2018xx2018+1)`
`=1`
Câu `5`
`4/9+X=7/9`
`<=>X=7/9-4/9`
`<=>X=3/9`
`<=>X=1/3`
Vậy `X=1/3`