1 -x^2+6x-10=-(x^2-6x+9+1)=-(x-3)^2-1恒小于零2 4x^2-12x+10=(2x)^2-12x+9+1=(2x-3)^2+1恒大于零3 x^2+y^2+x^2y^2-6xy+5=(x^2+y^2-2xy)+(xy)^2-4xy+4 +1 =(x-y)^2+(xy-2)^2+1>0
