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2020-04-02 09:36:11
期望值=15%*40%+10%*60%=12%标准差=[40%*(12%-15%)^2+60%*(12%-10%)^2]^(1/2)=(0.4*0.0009+0.6*0.0004)^(1/2)=0.0006^(1/2)=0.0245
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