∫ye^（-y）dy
=-∫ye^（-y）d(-y)
=-∫yde^(-y)
=-ye^(-y)+∫e^(-y)dy
=-ye^(-y)-∫e^(-y)d(-y)
=-ye^(-y)-e^(-y)
=-(y+1)/e^y
x趋于无穷
-(y+1)/e^y是无穷/无穷
可以用罗比达法则求极限
=-1/e^y
所以极限=0
x=0,-(y+1)/e^y=-1
所以原式=0-(-1)=1