Bài 2: (2 điểm) Tìm x biết: a)  $\frac{1}{3}x$ + $\frac{2}{5}(x-1)= 0$  b)  $3. (3x - \frac{1}{2})^2$ + $\frac{1}{9}= 0$  c)  $12,3 :x  - 4, 5: x = 15$ d) $\frac{3- x}{5-x}$ = $(\frac{-3}{5})^2$ 

Giải thích các bước giải:

`a) 1/3x + 2/5 ( x - 1 ) = 0`

`1/3x + 2/5x - 2/5 = 0`

`( 1/3 + 2/5 )x - 2/5 = 0`

`( 5/15 + 6/15 )x - 2/5 = 0`

`11/15x - 2/5 = 0`

`11/15x = 2/5`

`x = 2/5 : 11/15`

`x = 2/5 . 15/11`

`x = 6/11`

Vậy, `x = 6/11`

`-----`

`b) 3 ( 3x - 1/2)^3 + 1/9 = 0`

`3( 3x - 1/2)^3 = -1/9`

`( 3x - 1/2)^3 = -1/9 : 3`

`( 3x - 1/2)^3 = -1/9 . 1/3`

`( 3x - 1/2)^3 = -1/27`

`( 3x - 1/2)^3 = (-1/3)^3`

`3x - 1/2 = -1/3`

`3x = -1/3 + 1/2`

`3x = -2/6 + 3/6`

`3x = 1/6`

`x = 1/6 : 3`

`x = 1/6 . 1/3`

`x = 1/18`

Vậy, `x = 1/18`

`-----`

`c) 12,3 : x - 4,5 : x = 15`

`( 12,3 - 4,5 ) : x = 15`

`7,8 : x = 15`

`x = 7,8 : 15`

`x = 0,52`

Vậy, `x = 0,52`

`------`

`d) (3 - x)/(5 - x) = (-3/5)^2`

`( 3 - x)/(5 - x) = 9/25`

`25 ( 3 - x) = 9 (5 - x )`

`75 - 25x = 45 - 9x`

`75 - 25x - 45 + 9x = 0`

`( 75 - 45 ) + ( -25x + 9x ) = 0`

`30 - 16x = 0`

`16x = 30`

`x = 30/16`

`x =15/8`

Vậy,`x=15/8`

`#By`