Tìm x, biết: a) $\frac{x}{3}$ = $\frac{-4}{6}$ b) $\frac{x - 1}{3}$ = $\frac{8}{6}$ c) $\frac{x}{9}$ = $\frac{4}{x}$ d) $\frac{x + 1}{3}$ = $\frac{3}{x + 1}$

`a, x/3 = (-4)/6`

`=> 6x = -4. 3`

`=> 6x = -12`

`=> x = -12 : 6`

`=> x = -2`

Vậy `x = -2`

`b, (x-1)/3 = 8/6`

`=> (x-1)/3 = 4/3`

`=> 3(x-1) = 4.3`

`=> 3x - 3 = 12`

`=> 3x = 12+3`

`=> 3x = 15`

`=> x = 15 : 3 = 5`

Vậy `x = 5`

`c, x/9 = 4/x`

`=> x.x = 4.9`

`=> x^2 = 36`

`=> x^2 = (±6)^2`

`=> x = ±6`

Vậy `x in {6;-6}`

`d, (x+1)/3 = 3/(x+1)`

`=> (x+1)(x+1) = 3. 3`

`=> (x+1)^2 = 9`

`=> (x+1)^2 = (±3)^2`

`=> [(x+1=3),(x+1=-3):}`

`=> [(x=2),(x=-4):}`

Vậy `x in {2'-4}`