a=0,
f(x)=x^3在[0,2}上单增
f max=f(2)=8
a≠0
f'=3x^2-2ax=0
x=0 or x=2a/3
a>0
2a/3>=2,a>=3,
f(x)在[0,2}上单减
f max=f(0)=0
0