x^2+xy=2 y^2+xy=7,则2(x-y)(x+y)=
问题描述:
x^2+xy=2 y^2+xy=7,则2(x-y)(x+y)=
答
x^2+xy=2--------1式
y^2+xy=7---------2式
用1式减去2式有x^2-y^2=(x-y)(x+y)=-5