第2个回答 2019-10-31
(1)
∫(0->+∞) xe^[-(x+y)] dy
=xe^(-x) . ∫(0->+∞) e^(-y) dy
=-xe^(-x) . [ e^(-y) ]|(0->+∞)
=xe^(-x)
(2)
∫(0->+∞) xe^[-(x+y)] dx
=e^(-y). ∫(0->+∞) xe^(-x) dx
=-e^(-y). ∫(0->+∞) x de^(-x)
=-e^(-y). [ xe^(-x)]|(0->+∞) + e^(-y). ∫(0->+∞) e^(-x) dx
=0 -e^(-y). [ e^(-x)]|(0->+∞)
=e^(-y)本回答被提问者和网友采纳