f(x)=ax(x-2)^2=ax^3-4ax^2+4ax
f'(x)=3ax^2-8ax+4a=3a(x-2/3)(x-2)
讨论:(1)a>0,
∵x∈(-∞,2/3）,f'(x)>0,x∈(2/3,2）,f'(x)