f'(x) = 3ax²-3,令之为0,得 x²=1/a ,x=±1/√a
判断符号可知,f(x)的极小点为 x = 1/√a
∴ f(-1) = -a+4 ≥ 0
f(1) = a-2 ≥ 0
f(1/√a) = 1/√a -3/√a +1 ≥ 0
得 a ≤ 4 ,a ≥ 2 ,a ≥ 4
∴ a = 4
f'(x) = 3ax²-3,令之为0,得 x²=1/a ,x=±1/√a
判断符号可知,f(x)的极小点为 x = 1/√a
∴ f(-1) = -a+4 ≥ 0
f(1) = a-2 ≥ 0
f(1/√a) = 1/√a -3/√a +1 ≥ 0
得 a ≤ 4 ,a ≥ 2 ,a ≥ 4
∴ a = 4