a, Đặt `x^2+3x+1=t`
Khi đó `A=t(t+1)-6`
`=t^2+t-6`
`=t^2-2t+3t-6`
`=t(t-2)+3(t-2)`
`=(t+3)(t-2)`
`=(x^2+3x+4)(x^2+3x-1)`
b, `B=(x+1)(x+3)(x+5)(x+7)+15`
`=(x^2+8x+7)(x^2+8x+15)+15`
Đặt `t=x^2+8x+7`
Khi đó `B=t(t+8)+15`
`=t^2+8t+16-1`
`=(t+4)^2-1`
`=(t+5)(t+3)`
`=(x^2+8x+12)(x^2+8x+10)`
`=[(x^2+8x+16)-4][(x^2+8x+16)-6]`
`=[(x+4)^2-4][(x+4)^2-6]`
`=(x+6)(x-2)(x+4-sqrt{6})(x+4+sqrt{6})`