P(x=1)=0.2，P(x=2)=0.8×0.4=0.32，P(x=3)=0.8×0.6×0.6=0.288，P(x=4)=0.8×0.6×0.4×0.8=0.1536，p(x=5)=0.8×0.6×0.4×0.2×1=0.0384，E(x)=1×0.2＋2×0.32＋3×0.288＋4×0.1536＋5×0.0384＝2.5104。满意就采纳吧！
0.2*1+0.8*0.4*2+0.8*0.6*0.6*3+0.8*0.6*0.4*0.8*4+0.8*0.6*0.4*0.2*5=2.5104
（1*0.2+2*0.4+3*0.6+4*0.8+5*1）/5=
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