1.4x^2-y^2+4y-4
=4x²-(y-2)²
=（2x+y-2)(2x-y+2)
2.a-a^3+4a^2b-4ab^2
=a[1-(a²-4ab+4b²)]
=a[1-(a-2b)²]
=a(1+a-2b)(1-a+2b)