112
,∴
13
(1?p)2 =
,∴p=
12
,p(x = 1)= 0
23
×
+
=
412
p(X = 2)= 1
512
,p(x=3)=1-
212
,∴EX=1×
+2×
+3×
53
所以答案是:53。