I1 = (-x-1) e^(-x)|(x=0) - (-x-1) e^(-x)|(x=1) = 2/e -1
I2 = ( x-3) e^(-x)|(x=2) - ( x-3) e^(-x)|(x=1) = 2/e - e^(-2)
I1 - I2 = e^(-2) - 1-x-1 和x-3是怎么来的呢分部积分∫ x e^(-x) dx = -∫ x d ( e^(-x)) = - x e^(-x) + ∫ e^(-x) dx = (-x-1) e^(-x) + C……