由对称轴-b/2a=1/3a=-3b/2当x=2时,y.>04a+2b+c>0-6b+2b+c>即 c-4b>0
能 b/-2a=1/3 b=-2/3a a+2/3a+c>0 5/3a+c>0 -2/5b+c>0 -2b+5c>0 -4b+10c>0 c<0 c-4b>0
