分解因式。急急急！！

1:为清楚起见，令x+y=m，xy=n
则原式=(m-2n)(m-2)+(n-1)^2
=m^2-2m-2mn+4n+n^2-2n+1
=(m-n)^2-2m+2n+1
=(m-n)^2-2(m-n)+1
=(m-n-1)^2
=(x+y-xy-1)^2
=(xy-x-y+1)^2
=(x-1)^2(y-1)^2

2、(x+1)(x+2)(x+3)(x+6)+x^2
=[(x+1)(x+6)][(x+2)(x+3)]+x^2
=[(x^2+6)+7x][(x^2+6)+5x]+x^2
=(x^2+6)^2+12x(x^2+6)+35x^2+x^2
=(x^2+6)^2+12x(x^2+6)+36x^2
=(x^2+6x+6)^2

(x+y-2xy)(x+y-2)+(xy-1)^2
=[(x+y)-2xy][(x+y)-2]+(xy-1)^2
=(x+y)^2-2(xy+1)(x+y)+4xy+x^2y^2-2xy+1
=(x+y)^2-2(xy+1)(x+y)+x^2y^2+2xy+1
=(x+y)^2-2(xy+1)(x+y)+(xy+1)^2
=[(x+y)-(xy+1)]^2
=(x+y-xy-1)^2
=[x(1-y)-(1-y)]^2
=(x-1)^2(1-y)^2
=(x-1)^2(y-1)^2

(x+1)(x+2)(x+3)(x+6)+x^2
=[(x+1)(x+6)][(x+2)(x+3)]+x^2
=[(x^2+6)+7x][(x^2+6)+5x]+x^2
=(x^2+6)^2+12x(x²+6)+35x^2+x^2
=(x^2+6)^2+12x(x^2+6)+36x^2
=(x^2+6x+6)^2