先化简:(xy-2y)(xy+2y)-(x-y)2-x2y2,并求当x=1/2、y=2时的代数式的值.

问题描述:

先化简:(xy-2y)(xy+2y)-(x-y)2-x2y2,并求当x=

1
2
、y=2时的代数式的值.

原式=x2y2-4y2-(x2-2xy+y2)-x2y2
=x2y2-4y2-x2+2xy-y2-x2y2
=-5y2-x2+2xy,
当x=

1
2
,y=2时,原式=-20-
1
4
+2=-18
1
4