已知x,y∈R,求证:x+y+1≥x+y+xy
问题描述：
已知x,y∈R,求证:x+y+1≥x+y+xy
答
2x+2y+2≥2x+2y+2xy x+x+y+y-2x-2y+1+1-2xy≥0 (x+1)+(y+1)+(x-y)≥0 平方都≥0所以成立
已知x,y∈R,求证:x+y+1≥x+y+xy
2x+2y+2≥2x+2y+2xy x+x+y+y-2x-2y+1+1-2xy≥0 (x+1)+(y+1)+(x-y)≥0 平方都≥0所以成立