2000^2-1999*2001
=2000^2-1999*2000-1999
=2000*(2000-1999)-1999
=2000*1-1999
=1
(x+y)^2-(x-y)^2
=4xy
=7-5
=2
xy=2/4
=1/2
(x+y)^2-(x^2+y^2)
=2xy
=4-15
=-11
xy=-11/2
请点击下面的【选为满意回答】按钮,若x+y=2,x^2+y^2=15,求xy的值看答案！更新了！