x2-2xy+y2+4x-4y+4=(x-y)2+4(x-y)+4=(x-y+2)2
（x+y)2-4(x+y-1)=(x+y)2-4(x+y)+4=(x+y-2)2
x3+4x2+x-6=x(x2+4x+1+3-3)-6=x(x2+4x+4)-3x-6=x(x+2)2-3(x+2)=(X-3)(x+2)2