即f'(x)=4x^3-3ax^2+2x=x(4x^2-3ax+2)=0仅有一个实根,
则4x^2-3ax+2=0的Δ≤0(可以等于0,因为在0点两侧f'(x)符号不变,单调性不变),
a∈[-4√2/3,4√2/3]