证明：作差得
x^2-2x+5-(4y-y^2)
=x^2-2x+1+4-4y+y^2
=x^2-2x+1+y^2-4y+4
=(x-1)^2+(y-2)^2
>=0
所以x^2-2x+5>=4y-y^2
注：“>=”代替大于等于