试比较x2+y2与xy+x+y-1的大小.

问题描述:

试比较x2+y2与xy+x+y-1的大小.

∵x2+y2-(xy+x+y-1)
=x2-(y+1)x+y2-y+1
=(x−

y+1
2
)2
1
4
(y+1)2+y2−y+1
=(x−
y+1
2
)2+
3
4
(y−1)2
≥0(10分)
∴x2+y2≥xy+x+y-1