`a)``x^2-8x+7=0`

`<=>x^2-x-7x+7=0`

`<=>x(x-1)-7(x-1)=0`

`<=>(x-7)(x-1)=0`

`<=>[(x-7=0),(x-1=0):}`

`<=>[(x=7),(x=1):}`

Vậy `x in {7;1}`

`b)``x^2-4x-5=0`

`<=>x^2+x-5x-5=0`

`<=>x(x+1)-5(x+1)=0`

`<=>(x-5)(x+1)=0`

`<=>[(x-5=0),(x+1=0):}`

`<=>[(x=5),(x=-1):}`

Vậy `x in {5;-1}`

`c)``x^2-4x+3=0`

`<=>x^2-x-3x+3=0`

`<=>x(x-1)-3(x-1)=0`

`<=>(x-3)(x-1)=0`

`<=>[(x-3=0),(x-1=0):}`

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

Vậy `x in {3;1}`

`d)``x^2+5x+4=0`

`<=>x^2+x+4x+4=0`

`<=>x(x+1)+4(x+1)=0`

`<=>(x+4)(x+1)=0`

`<=>[(x+4=0),(x+1=0):]`

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

Vậy `x in {-4;-1}`