概车纶与款理统外 贿4x-(品=-1( 20 20 i=1,2,.,10. 由此=1-(8，= 20 得E(X)=E(X1+X2+.+Xo) =E(X1)+E(X2)+.+E(X0) =1-(门-4, 10 9 { 0} 20       则有 P Xi = = , 10 9 { 1} 1 20       P Xi = = − i = 1,2,  ,10. , 1,2, . 10 9 ( ) 1 2 0  =       E X = − i 由此 i ( ) ( ) 得 E X = E X1 + X2 ++ X10 ( ) ( ) ( ) = E X1 + E X2 ++ E X1 0             = − 20 10 9 10 1 = 8.784(次)