已知；（x+y）²=25（x-y）²=9 求xy与x²+y²的值
问题描述：
已知；（x+y）²=25（x-y）²=9 求xy与x²+y²的值
要求有过程,最好附加详解
答
∵(x+y)²=25
(x-y)²=9
∴xy
=[(x+y)²-(x-y)²]/4
=(25-9)/4
=16/4
=4
∴x²+y²
=(x+y)²-2xy
=25-2×4
=25-8
=17为什么要/4啊(x+y)²-(x-y)²=x²+2xy+y²-(x²-2xy+y²)=4xy