B-27ke=2g e7kzega 2 4 (k/2+1)(k/2)T(k/2)=k2+2k T(k/2) D(X)=EX2)-E(X)2=k2+2k-k2=2k. 函国D☒( ) ( ) [ ( )] . ( / ) ( / ) ( / ) ; ( / ) ( ) ( / ) / ( / ) ( / ) ( ) / / / / / / / / D X E X E X k k k k k k k k k k t e dt k t x x e dx k x e dx k E X k t k k x k k x k 2 2 2 1 2 2 2 2 4 2 2 2 2 2 2 2 1 2 2 1 2 2 2 2 2 0 2 1 2 0 0 2 1 2 2 2 1 2 2 2 = − = + − = =  +   = + = = =    + + − +  +  + − + −     