若(x-2y)^2=5,(x+2y)^2=9,求xy的值

问题描述:

若(x-2y)^2=5,(x+2y)^2=9,求xy的值

(x-2y)^2-(x+2y)^2=5-9
(x-2y+x+2y)(x-2y-x-2y)=-4
(2x)(-4y)=-4
-8xy=-4
xy=1/2