S(x e^-x dx)
=S(-x d(e^-x))
=-x e^-x - S(e^-x d-x)
=-x e^-x + S(e^-x dx)
=-x e^-x - e^-x
=-(e^-x) (1+x)
将区间代入
-(e^-1)(1+1)+(e^1)(1-1)=-2e^-1