a,
`x^3+8x^2+11x-20`
`=x^3+5x^2+3x^2+15x-4x-20`
`=x^2(x+5)+3x(x+5)-4(x+5)`
`=(x+5)(x^2+3x-4)`
`=(x+5)(x-1)(x+4)`
b,
`(x-2)(x-1)(x+4)(x+5)-125`
`=(x-2)(x+5)(x-1)(x+4)-125`
`=(x^2+3x-10)(x^2+3x-4)-125`
Đặt `x^2+3x-7=t`
`Pt=(t-3)(t+3)-125`
`=t^2-9-125`
`=t^2-134`
`=(x^2+3x-7-√134)(x^2+3x-7+√134)`