`B6,`

`a,y = (2x - 1)/(1 - x)`

`Đk:  1 - x ne\ 0=>xne\ 1`

`b,y = 1/(x^2 + 4x + 5)`

`Đk: x^2 + 4x + 5 = (x + 2)^2 + 1 > 0 ,∀ x(x ∈ R)`

`c,y = (2x - 1)/(x^2 - 3x + 2)`

`Đk: x^2 - 3x + 2 = (x - 1)(x - 2) ne\ 0=>x ne\ 1, x ne\ 2`

`d,y = (x + 2)/(x^2 - 5)`

`Đk:x^2 - 5 = (x - sqrt(5))(x + sqrt(5)) ne\0=>x ne\+- sqrt(5)`

`e,y = (2x)/(x^2 - 4x - 5)`

`Đk: x^2 - 4x - 5 = (x - 5)(x + 1) ne\0=> x ne\ 5, x ne\ -1`

`B7,`

`a,y = 2/(4x - 9) + x/(x + 1)>0`

`Đk: 4x - 9 ne\ 0, x + 1 ne\ 0=>x ne\9/4, x ne\ -1`

`b,y = -5/(4x^2 - 9)`

`ĐK :4x^2 - 9 = (2x - 3)(2x + 3) ne\ 0=>x ne\ 3/2, x ne\ -3/2`

`d,y = (2x)/(x^2 - 3x - 4)`

`Đk: x^2 - 3x - 4 = (x - 4)(x + 1) ne\ 0=>x ne\ 4, x ne\ -1`

`e,y = (2x)/(|x| - 9)`

`Đk: |x| - 9 ne\  0=>x ne\+- 9`

`f,y = (-3x)/(x^2 - 10x + 16)`

`Đk:x^2 - 10x + 16 = (x - 2)(x - 8) ne\ 0=>x ne\ 2, x ne\ 8`

`@612009`