f'(x)=3x^2-2ax

=x(3x-2a)>0

Incremental function, the minimum value is f (1) =1-a.

② x 2 > when 1 < a ≤ 2; 0, |x-a|≥0 Minimum value is 0.

③a & gt； 2 f(x)=x^2(a-x)=-x^3+ax^2

f'(x)=-3x^2+2ax

=x(2a-3x)

3≤3x≤62a & gt； four

③-Ⅰ: When 4

The minimum value of f(x) f (2a/3) = 4a 3/27.

③-Ⅱ: when a >; 32a & gt； 6，3≤3x≤6

f′(x)= x(2a-3x)>0

The minimum value of f(x) f( 1) =a- 1.