`a)x^3-7x+6=0`

`(x^3-x)+(-6x+6)=0`

`x(x^2-1)-6(x-1)=0`

`x(x+1)(x-1)-6(x-1)=0`

`(x-1)[x(x+1)-6]=0`

`(x-1)(x^2+x-6)=0`

`(x-1)[(x^2-2x)+(3x-6)]=0`

`(x-1)[x(x-2)+3(x-2)]=0`

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

`x=1` hoặc `x=-3` hoặc `x=2`

Vậy:` S={1;-3;2}`

`b)x^4-4x^3+12x-9=0`

`(x^4-x^3)+(-3x^3+3x^2)+(-3x^2+3x)+(9x-9)=0`

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

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

`(x-1)[x^2(x-3)-3(x-3)]=0`

`(x-1)(x^2-3)(x-3)=0`

`x=1` hoặc `x^2=3` hoặc `x=3`

`x=1` hoặc `x=+-\sqrt{3}` hoặc `x=3`

Vậy:` S={1;\sqrt{3};-\sqrt{3};3}` 

`c)x^5-5x^3+4x=0`

`x(x^4-5x^2+4)=0`

`x[(x^4-x^3)+(x^3-x^2)+(-4x^2-4)]=0`

`x[x^3(x-1)+x^2(x-1)-4(x+1)(x-1)]=0`

`x(x-1)[x^3+x^2-4(x+1)]=0`

`x(x-1)[x^2(x+1)-4(x+1)]=0`

`x(x-1)(x^2-4)(x+1)=0`

`x(x-1)(x+2)(x-2)(x+1)=0`

`x=0` hoặc `x=1` hoặc `x=-2` hoặc `x=2` hoặc `x=-1`

Vậy:` S={0;1;-2;2;-1}` 

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

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

`x^3(x-1)-3x^2(x-1)-4(x-1)=0`

`(x^3-3x^2-4)(x-1)=0`

`[(x^3+x^2)-4(x^2-1)](x-1)=0`

`[x^2(x+1)-4(x+1)(x-1)](x-1)=0`

`(x^2-4(x-1))(x+1)(x-1)=0`

`(x^2-4x+4)(x+1)(x-1)=0`

`(x-2)^2(x+1)(x-1)=0`

`x=2` hoặc `x=-1` hoặc `x=1`

Vậy:` S={2;-1;1}` 

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