5x^2+y^2+z^2－（2xy+4x+2z-2）＝4x^2－4x+1+x^2+y^2-2xy+z^2-2z+1=(2x-1)^2+(x+y)^2+(z-1)^2>=0
所以5x^2+y^2+z^2大于等于2xy+4x+2z-2
比较大小 5x^2+y^2+z^2与2xy+4x+2z-2
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