$#DAnh$

`d.`

`3(2x + 3) - 7 = 20` 

`6x + 9 - 7 = 20`

`6x + 2 = 20`

`6x = 18`

`x = 3`

Vậy `S = {3}` .

`h.`

`12 - (5x - 4) = 2x - 5`

`12 - 5x + 4 = 2x - 5`

`16 - 5x = 2x - 5`

`16 + 5 = 2x + 5x`

`21 = 7x`

`x = 3`

Vậy `S = {3}` .

`l.`

`(x + 4)^2 - (x - 2)^2 = 36`

`(x^2 + 8x + 16) - (x^2 - 4x + 4) = 36`

`x^2 + 8x + 16 - x^2 + 4x - 4 = 36`

`12x + 12 = 36`

`12x = 24`

`x = 2`

Vậy `S = {2}` .

`q.`

`9x^2 - 16 = 0`

`(3x)^2 - 4^2 = 0`

`(3x - 4)(3x + 4) = 0` 

`3x - 4 = 0` hoặc `3x+4=0`

`3x = 4` hoặc `3x=-4` 

`x = 4/3` hoặc `x = -4/3`

Vậy `S = {4/3; -4/3}` .

`u.`

`x^2 - 8x + 12 = 0`

`x^2 - 2x - 6x + 12 = 0` 

`x(x - 2) - 6(x - 2) = 0` 

`(x - 2)(x - 6) = 0`

`x - 2 = 0` hoặc `x-6=0`

`x = 2` hoặc `x = 6`

Vậy `S = {2; 6}` .