已知x+y=3,xy=-7.求(x-y)^2的值
问题描述：
已知x+y=3,xy=-7.求(x-y)^2的值
答
(x-y)^2
=x²-2xy+y²
=x²+2xy+y²-4xy
=(x+y)²-4xy
=3²-4×(-7)
=37