已知x-2y=5,xy=1.求下列各式的值: (1)2x2y-4xy2 (2)-x2+4xy-4y2.

问题描述:

已知x-2y=5,xy=1.求下列各式的值:
(1)2x2y-4xy2
(2)-x2+4xy-4y2

(1)原式=2xy(x-2y)                             
=2×1×5=-52
=10=-25
(2)原式=-(x-2y)2=-25