f（x）＝2x3－3ax2＋a+b
f '（x）＝6x2－6ax=0
x=0 or x=a
函数f(x)的单调区间；(-∞,0)(0,a)(a,+∞) (a>0)
函数f(x)的单调区间；(-∞,a)(a,0)(0,+∞) (a