Bài 2:
a) `2/5 + ( 3/11 + (-2)/5)`
`= 2/5 + 3/11 + (-2/5)`
`= (-2/5 + 2/5) + 3/11`
`= 3/11`
b) `-5/7 . 4/13 + (-5)/7 . 9 /13 + (-2/7)`
`= -5/7 ( 4/13 + 9/13) + (-2/7)`
`= -5/7 . 1 +(-2/7)`
`= -5/7 + (-2/7)`
`= -7/7`
` =-1`
Bài 3:
a) `(2,8 x - 32): 2/3= = -90`
`=> 2,8 x - 32 = -90 . 2/3`
`=> 2,8 x - 32 = -60`
`=> 2,8 x = -60 + 32`
`=> 2,8 x = -28`
`=> x= -28 : 2,8`
`=> x =-10`
Vậy `x= -10`
b) `4/5 + 5/7 : x= = 1/6`
`=> 5/7 : x = 1/6 - 4/5`
`=> 5/7 : x = -19/30`
`=> x = 5/7 : (-19)/30`
`=> x= 5/7 . (-30)/19`
`=> x= -150/133`
Vậy `x= -150/133`