已知a∈R,函数f(x)=-1/3x^3+1/2ax^2+2ax(a∈R) (1)当a=1时,求函数f(x)的单调递增区间; (2)若函数f(x)在R上单调递减,求a的取值范围; (3)若函数f(x)在[-1,1]上单调递减,求a的取值范围