xx
xx0,x^2+x+1=(x+1/2)^2+3/4>0y=3x/(x^2+x+1)
y=3x/(x^2+x+1)=3/(x+1/x+1)因为x<0所以x+1/x=-((-x)+1/(-x))<=-2 y=3x/(x^2+x+1)>=3/(-2+1)=-3