2(x-1)(x-9)
=2(x^2-10x+9)
=2x^2-20x+18
2(x-2)(x-4)
=2(x^2-6x+8)
=2x^2-12x+16
原多项式应为:2x^2-12x+18
=2(x^2-6x+9)
=2(x-3)^2