f(x)=x^3-3a^2*x+3(a+2)x+1
求导
f'(x)=3x²-6ax+3(a+2)
既有极大值又有极小值
所以
判别式△=36a²-36(a+2)
=36(a²-a-2)
=36(a-2)(a+1)>0
得 a>2 或 a