请教几道因式分解题！要过程（重要参考根据）！

1.-2xy-x的平方-y的平方(^代表次方）
-2xy-x^2-y^2
=-(2xy+x^2+y^2)
=-(x+y)^2
2.原式=（2x-1）^2

1. -2xy-x^2-y^2=-(x^2+2xy+y^2)=-(x+y)^2
2. 4x^2-4x+1=(2x)^2-4x+1=(2x-1)^2
3. -3x^2+6xy-3y^2=-(3x^2-6xy+3y^2)=-3(x^2-2xy+y^2)=-3(x-y)^2
4.(a-b)^2-1=(a-b+1)(a-b-1)
5.

(1)-2xy-x^2-y^2
=x^2+2xy+y^2
=x^2+2*x*y+y^2
=(x+y)^2

(2)4x^2-4x+1
=(2x)^2-4x+1
=(2x-1)^2

(3)-3x^2+6xy-3y^2
=-(3x^2-6xy+3y^2)
=-3(x^2-2xy+y^2)
=-3(x-y)^2

(4)(a-b)^2-1
=(a-b+1)(a-b-1)

(5)(19.98)^2-(20.98)^2
=(19.98+20.98)(19.98-20.98)
=40.96(-10)
=-409.6

(6)1000/252^2-248^2
=(1000/252+248)(1000/252-248)
(第6题是因式分解.不是计算所以楼上的全错了）

5.(19.98)^2-(20.98)^2
=(19.98+20.98)(19.98-20.98)
=40.96(-10)
=-409.6
6.

1.1.-2xy-x的平方-y的平方(^代表次方）
-2xy-x^2-y^2
=-(2xy+x^2+y^2)
=-(x+y)^2
2.原式=（2x-1）^2
3. -3x^2+6xy-3y^2=-(3x^2-6x