`d)`
`x^7+x^5+1`
`=(x^7+x^6+x^5)-(x^6-1)`
`=x^5(x^2+x+1)-(x^3-1)(x^3+1)`
`=x^5(x^2+x+1)-(x-1)(x^2+x+1)(x^3+1)`
`=(x^2+x+1)[x^5-(x-1)(x^3+1)]`
`=(x^2+x+1)(x^5-x^4-x+x^3+1)`
`e)`
`(x-1)(x-2)(x-3)(x-4)-80`
`=[(x-1)(x-4)][(x-2)(x-3)]-80`
`=(x^2-x-4x+4)(x^2-3x-2x+6)-80`
`=(x^2-5x+4)(x^2-5x+6)-80`
Đặt `t=x^2-5x+4`
`=t(t+2)-80`
`=t^2+2t-80`
`=t^2+10t-8t-80`
`=t(t+10)-8(t+10)`
`=(t-8)(t+10)`
`=(x^2-5x+4-8)(x^2-5x+4+10)`
`=(x^2-5x-4)(x^2-5x+14)`