Bài 1 : a) `-2/5+7/8`
`=-16/40+35/40`
`=19/40`
b) `2/3-4:(1/2+3/4)`
`=2/3-4:(2/4+3/4)=2/3-4:5/4`
`=2/3-16/5`
`=10/15-48/15`
`=-38/15`
c) `(-2/3)^3.sqrt81+4/3`
`=(-2^3/3^3).9+4/3`
`=(-8/27).9+4/3`
`=[(-8)/27+4/3].9`
`=[(-8)/27+36/27].9`
`=28/27.9=28/27.9/1`
`=28/3`
d)
Bài 2 : a) `2/3x+5/7=-3/10`
`2/3x=-3/10-5/7=21/70-50/70`
`2/3x=-29/70`
`x=-29/70:2/3=-29/70×3/2`
`=87/140`
b) `(2x-1)^3=8`
`(2x-1)^3=2^3`
`(2x-1)=2`
`2x-1=2`
`2x=2+1`
`2x=3`
`x=3/2`
Bài 3 :
`x/y=-3/5` và `y-x=24`
`\Rightarrowx/5=y/-3` và `y-x=24`
Áp dụng t/c của dãy tỉ số = nhau :
`x/5=y/-3=y-x/(-3)-5=24/-8=-3`
Do đó :
$\Rightarrow\dfrac{x}{5}=-3\Rightarrow{x=-3.5=-15}$
$\Rightarrow\dfrac{y}{-3}=-3\Rightarrow{y=-3.-3=9}$
