1.Tìm x , biết : a) x-21=-5 b)-25-x=17 c)2.(x+3)-5=7 d)17-3(x+4)=8 e)5-(-2x+1)=12 f)15-2(2x-1+5

`a) x-21=-5`

`=> x=-5+21`

`=> x=16`

Vậy `x=16`

`-------`

`b) -25-x=17`

`=> x=-25-17`

`=> x=-42`

Vậy `x=-42`

`---------`

`c) 2(x+3)-5 = 7`

`=> 2(x+3)=7+5`

`=> 2(x+3) = 12`

`=> x+3 = 12:2`

`=> x+3=6`

`=> x=6-3`

`=> x=3`

Vậy `x=3`

`------------`

`d) 17-3(x+4)=8`

`=> 3(x+4)=17-8`

`=> 3(x+4)=9`

`=> x+4=9:3`

`=> x+4=3`

`=> x=3-4`

`=> x=-1`

`----------`

`e) 5-(-2x+1) = 12`

`=> -2x+1 = 5-12`

`=> -2x+1 = -7`

`=> -2x = -7-1`

`=> -2x=-8`

`=> x=-8:(-2)`

`=> x=4`

Vậy `x=4`

`------------`

`f) 15-2(2x-1)=5`

`=> 2(2x-1)=15-5`

`=> 2(2x-1)=10`

`=> 2x-1=10:2`

`=> 2x-1=5`

`=> 2x=5+1`

`=> 2x=6`

`=> x=6:2`

`=> x=3`

Vậy `x=3`