a³-4a
=a(a²-4)
=a(a+2)(a-2)
x³-2x²y+xy²
=x(x²-2xy+y²)
=x(x-y)²
-x²+4x-4
=-(x²-4x+4)
=-(x-2)²
x²y-5xy²+6y³
=y(x²-5xy+6y²)
=y(x-2y)(x-3y）