根據(jù)y邊緣密度函數(shù)
fY(y)=∫0+∞（就是0到正無窮的積分 下面一樣）(乘以)f(x,y)dx
得
當(dāng)y>0時(shí) 有
f(y)=∫0+∞e^[-(x+y)]dx= e^[-y]∫0+∞e^[-x]dx= e^[-y](-e^[-x]∣0+∞)= e^[-y](0+1)= e^[-y]
當(dāng)y<=0時(shí) f(y)=0