求证：ｘ＾２+ｙ＾２+１≥ｘ+ｙ+ｘｙ
令Z=ｘ＾２+ｙ＾２+１-(ｘ+ｙ+ｘｙ)= ｘ＾２-(1+y)ｘ+ｙ^2+1-y△=-3(y-1)^2≤0,所以Z≥0,所以 ｘ＾２+ｙ＾２+１≥ｘ+ｙ+ｘｙ 成立
