已知x+y=3,4xy=3.求（x-y）ˇ2的值
问题描述：
已知x+y=3,4xy=3.求（x-y）ˇ2的值
答
(x-y)^2
=x^2-2xy+y^2
=x^2+2xy+y^2-4xy
=(x+y)^2-4xy
=3^2-3
=9-3=6-4xy从哪里来的？我晕哦，-2xy=-4xy+2xy