对y积分时把x看成常数所以=∫x*e^(-y)dy+∫y*e^(-y)dy∫x*e^(-y)dy=-x*e^(-y)∫y*e^(-y)dy=-∫y*e^(-y)d(-y)=-∫yde^(-y)=-y*e^(-y)+∫e^(-y)dy=-y*e^(-y)-e^(-y)所以∫x*e^(-y)dy+∫y*e^(-y)dy=e^(-y)*(-x-y-1)y=1,...