Ch5-32 k 10 20 P 0.5 0.5 E(Xk)=15,D(X)=25 X1,X2,…X100相互独立同分布,X=∑Xk E(X)=1900×15=28500 D(X)=1900×25=47500 近似 X~N(28500,47500Ch5-32 Xk P 10 20 0.5 0.5 E(Xk ) =15, D(Xk ) = 25 = = 1900 k 1 1 2 1900 X Xk X , X ,  , X 相互独立同分布, ( ) 1900 25 47500 ( ) 1900 15 28500 =  = =  = D X E X X ~ N(28500,47500) 近似