f'(x)=3x²-9x+6=3(x²-3x+2)=3(x²-3x+9/4)-3/4=3(x-3/2)²-3/4≥-3/4
所以m的最大值为-3/4
f'(x)=3x²-9x+6=3(x²-3x+2)=3(x-1)(x-2)
f(x)在x=1处取极大值,在x=2处取极小值
∵f(x)=0有且仅有一个实根
∴f(1)0
f(1)0,即8-18+12-a>0解得a2.5或a