`4/5 : x = 2/7 - 1/5`

`4/5 : x = 10/35 - 7/35 = 3/35`

`x = 4/5 : 3/35 = 4/5 *35/3 = 28/3`

Vậy `x=28/3`

`120:(x+15:2)=140`

`x +15:2 = 120/140 = 6/7`

`x +15/2 = 6/7`

`x= 6/7 -15/2 = 12/14 - 105/14 `

`x =-93/14`

`x^2 -(2y)^2`

`=(x+2y)(x-2y)`

`(4x-1)^2 = 25*9`

`<=> (4x-1)^2 = 225`

`<=> [(4x-1=15),(4x-1=-15):}`

`<=> [(4x=15+1=16),(4x=-15+1=-14):}`

`<=> [(x=16/4 = 4),(x = -14/4 =-7/2):}`

Vậy `S={4;-7/2}`

`( x + 3/5 ) xx 6/12 = 2/3`

`(x+3/5 )xx 3/6 =2/3`

`x +3/5 = 2/3 xx 6/3 = 4/3`

`x =4/3 -3/5 = 11/15`

tìm x và y 

thì sao lại dùng hoặc vậy ông bạn :)

`xy -2x =0`

`<=> x(y-2) =0`

`<=> {(x=0),(y=2):}`

Vậy `x,y in {0;2}`

`sqrt{x-6} =2`

`Đk : x >=6`

`=> x-6 =4`

`=> x = 4+6 =10(t//m)`

Vậy `x=10`