(4). 9(x+y)2-4(x-y)2 (5). (x+y)2-4(x+y-1) (6). -x2y+2xy2-y3

(5x+y)(x+5y)
(x+y-2)2
-y(x-y)2

(4). 9(x+y)^2-4(x-y)^2
=[3(x+y)-2(x-y)][3(x+y)+2(x-y)]
=(x+5y)(5x+y);

(5). (x+y)^2-4(x+y-1)
=(x+y)^2-4(x+y)+4
=(x+y-2)^2;

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

解:
4). 9(x+y)^2-4(x-y)^2
=<3(x+y)+2(x-y)><3(x+y)-2(x-y)>
=(5x+y)(x+5y)
(5). (x+y)^2-4(x+y-1)
=(x+y)^2-4(x+y)+4
=(x+y-2)^2
(6). -x^2y+2xy^2-y^3
=-y(x^2-2xy+y^2)
=-y(x-y)^2

(4)5x平方+26xy+5y平方
(5)(x+y)2-4(x+y)+4=(x+y-2)2
(6)-y(x2-2xy+y2)=-y(x-y)2