x-y=3 x^2-y^2=18,x+y=_ xy=_

问题描述:

x-y=3 x^2-y^2=18,x+y=_ xy=_

x-y=3
x^2-y^2=(x+y)(x-y)=3(x+y)=18
所以x+y=6
xy=[(x+y)^2-(x-y)^2]/4=(6^2-3^2)/4=27/4