第二章****题 2 ¥ 1、设 X 为取非负整数值的随机变量,证明 E ( X )=åP (X ³ k ) k =1 证明: E(
X
)
¥ = åkp(x
=
k )
¥ = åk ( p( X
³
k )
-
p(
x
³ k +1)) k =1
k =1 ¥ åkp(x ³ k ) - å(k +1)p(x ³ k +1) + å p(x ³ k +1)¥ k =1 k =1 k =1 ¥ ¥ = p(k =1) + å p(x ³ k +1) = å p(x ³ k ) k =1 k =1 ìe - x , x > 0 = e aX (a < 0) 的数学期望。 2、设随机变量 X 的概率密度为 f (x) = í ,求Y î0, x £ 0 答: E(Y ) = E(eax ) = ò-+¥¥ eax f (x)dx = ò0+¥ eaxe-x dx = ò0+¥ ex(a-1) dx = 1 ò0+¥ de(a-1) x = 1 a -1 1- a ì6 xy 2 , 0 < x < 1, 0 < y <1 3、设二维随机变量(X,Y)的联合密度函数为 f (x , y ) = , í