x2+y2=7 xy+x+y=28
问题描述：
x2+y2=7 xy+x+y=28
求|x+y+1|的值
答
(x+y+1)^2
=(x+y)^2+2(x+y)+1
=x^2+y^2+2xy+2(x+y)+1
=(x^2+y^2)+2(xy+x+y)+1
=7+2*28+1
=64;
|x+y+1|=8