(I)∵=50∴a==0.5,b==0.3 (II)①依题意,随机选取一天,销售量为1.5吨的概率p=0.5 设5天中该种商品有X天的销售量为1.5吨,则X~B(5,0.5) P(X=2)=C52×0.52×(1-0.5)3=0.3125 ②ξ的可能取值为4,5,6,7,8,则 p(ξ=4)=0.22=0.04 p(ξ=5)═2×0.2×0.5=0.2 p(ξ=6)═0.52+2×0.2×0.3=0.37 p(ξ=7)═2×0.3×0.5=0.3 p(ξ=8)=0.32=0.09 所有ξ的分布列为
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