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.
答
∵x2+y2-(xy+x+y-1)
=x2-(y+1)x+y2-y+1
=(x−
)2−y+1 2
(y+1)2+y2−y+11 4
=(x−
)2+y+1 2
(y−1)2≥0(10分)3 4
∴x2+y2≥xy+x+y-1