Bài 1:
a) `-7/8 + 5/8`
`= -2/8`
`= -1/4`
b) `-15/63 . 8/21`
`= -5/21 . 8/21`
`=-40/441`
c) `-7/16 - 1/12`
`= -21/48 - 4/12`
`=-25/48`
d) `-7/15 . 5/8 . 15/-7 . (-16)`
`= 1 . 5/8 . (-16)`
`= 5/8 . (-16)`
`=-10`
Bài 2:
`x. 5/7 = 15/-21`
=> `x = 15/-21 : 5/7`
=> `x= 15/-21 . 7/5`
=> `x= -5/7 . 7/5`
=> `x= -1`
Vậy `x= -1`