设(X，Y)的概率密度为
f(x，y)=
{e-y，0≤x≤1，y＞0
{0，其他．
求E(X+Y)．
E(X+Y)=∫+∞-∞∫+∞-∞(x+y)f(x，y)dxdy=∫10(∫+∞0(x+y)(e-ydy)dx=∫10(x+1)dx=3/2
设(X，Y)的概率密度为
f(x，y)=
{e-y，0≤x≤1，y＞0
{0，其他．
求E(X+Y)．
E(X+Y)=∫+∞-∞∫+∞-∞(x+y)f(x，y)dxdy=∫10(∫+∞0(x+y)(e-ydy)dx=∫10(x+1)dx=3/2
