5.指数分布 x>0 x≤0 P E(X)=jxie=-jxde和 -xefe5.指数分布      = − 0 0 0 ( ) x e x f x x   1 = E X x e dx x   − = 0 ( )     − = − 0 x xde  x e e dx x x   −  − = − + 0 0  