已知(x+y)²=3,(x-y)²=1 求①x²+y² ②xy
问题描述：
已知(x+y)²=3,(x-y)²=1 求①x²+y² ②xy
答
(x+y)²=x²+2xy+y²,(x-y)²=x²-2xy+y²
所以有：
x²+y²=(x+y)²+(x-y)²
=3+1
=4
xy=[(x+y)²-(x-y)²]/4
=(3-1)/4
=1/2