（x^2+y^2)^2-4xy(x^2+y^2)+4x^2y^2
=(x²+y²-2xy)²
=(x-y)^4
1/2a^3x^2-2a^2xy+2ay^2
=1/2a(a²x²-4axy+4y²)
=1/2a(ax-2y)²(x^2-2x)^2-6(x^2-2x)+9会么(x^2-2x)^2-6(x^2-2x)+9=(x²-2x+3)=(x+1)(x-3)