`(x+1)(x+3)(x+5)(x+7)+15`
`=[(x+1)(x+7)].[(x+3)(x+5)]+15`
`=(x^2+8x+7)(x^2+8x+15)+15`
`text{Đặt}` `x^2+8+11=y.` `text{Biểu thức trở thành:}`
`(y-4)(y+4)+15`
`=y^2-16+15`
`=y^2-1`
`=(y+1)(y-1)`
`text{Thay lại.Ta được:}`
`(x^2+8x+12)(x^2+8x+10)`
`=(x^2+2x+6x+12).[x^2+(4+√6)x+(4-√6)x+10)`
`=[(x^2+2x)+(6x+12)].{[x^2+(4+√6)x]+[(4-√6)x+10]}`
`=[x(x+2)+6(x+2)].[x(x+4+√6)+(4-√6)(x+4+√6)]`
`=(x+6)(x+2)(x+4-√6)(x+4+√6).`
