x+y=3,xy=1,x^2+y^2=?
问题描述：
x+y=3,xy=1,x^2+y^2=?
答
x2+y2=（x+y）2-2xy=9-2*1=7
答
7 (x+y)^2-2xy=x^2+y^2=9-2=7
答
x^2+y^2
=x^2+y^2+2xy-2xy
=(x+y)²-2xy
=(3)²-2×1
=9-2
=7
答
(x+y)^2=3^2
x^2+2xy+y^2=9
x^2+y^2=9-2xy=9-2=7