a(x-3)^2=3b-1
整理
ax^2-6ax+9a-3b+1=0
∵有實(shí)數(shù)根
∴判別式△=36a^2-36a^2+12ab-4a=12ab-4a≥0
即a(3b-1)≥0
1°a=0,此時(shí)是-3b+1=0,是關(guān)于b的方程,所以舍去
2°a>0,開(kāi)口向上,△=a(3b-1)≥0
a>0,3b-1≥0,即b≥1/3
3°a