`13, x^4-4x^3-8x^2+8x`
`=x(x^3-4x^2-8x+8)`
`=x(x^3+2x^2-6x^2-12x+4x+8)`
`=x[x^2(x+2)-6x(x+2)+4(x+2)]`
`=x(x+2)(x^2-6x+4)`
`14, x^3-3x^2-3x+1`
`=x^3+x^2-4x^2-4x+x+1`
`=x^2(x+1)-4x(x+1)+(x+1)`
`=(x+1)(x^2-4x+1)`
`15, x^3+x^2-4x-4`
`=x^2(x+1)-4(x+1)`
`=(x+1)(x^2-4)`
`=(x+1)(x-2)(x+2)`
`16, x^4+x^3+x^2-1`
`=x^3(x+1)+(x-1)(x+1)`
`=(x+1)(x^3+x-1)`
`17, x^4-x^2+2x-1`
`=x^4-(x^2-2x+1)`
`=x^4-(x-1)^2`
`=(x^2-x+1)(x^2+x-1)`
`18, x^3-4x^2+4x-1`
`=x^3-x^2-3x^2+3x+x-1`
`=x^2(x-1)-3x(x-1)+(x-1)`
`=(x-1)(x^2-3x+1)`
