而与与n的边缘分布及数学期望为 0 2 P 5/12165/12 77 0 1/3 P7/121/121/3 5105 1113 则E= E7= 121212 36336 cov(S, n=E(Sn-eSEn 13513 221 3612364325 而x与h的边缘分布及数学期望为: x -1 0 2 P 5/12 1/6 5/12 h 0 1/3 1 P 7/12 1/12 1/3 432 221 36 13 12 5 36 13 cov( , ) ( ) 36 13 3 1 36 1 , 12 5 12 10 12 5 = - -  = - = - = = - + = = + = x h x h x h x h E E E 则E E