17, (x+2)(x+3)(x+4)(x+5)-24`
=[(x+2)(x+5)][(x+3)(x+4)]-24`
=(x^2+7x+10)(x^2+7x+12)-24 (1)`
Đặt : `t=x^2+7x+10`
`⇒(1)` có dạng : `t(t+2)-24`
`=t^2+2t-24`
`=t^2+6t-4t-24`
`=t(t+6)-4(t+6)`
`=(t-4)(t+6)`
`=(x^2+7x+10-4)(x^2+7x+10+6)`
`=(x^2+7x+6)(x^2+7x+16)`
`19, x^4-4`
`=(x^2)^2-2^2`
`=(x^2-2)(x^2+2)`