已知实数a>0,求函数f(x)=[ax^2+3(a+1)x+3a+6] / (e^x)的极值

f(x)=[ax^2+3(a+1)x+3a+6] / (e^x)f ‘(x)=｛[2ax+3（a+1）](e^x) - [ax^2+3(a+1)x+3a+6](e^x)｝/[e^（2x）]=｛[2ax+3（a+1）] - [ax^2+3(a+1)x+3a+6]｝/（e^x）=[- ax^2-(a+3)x-3]/（e^x）=[(ax-3)(-x+1)]/（e^x）...