英语周报八年级新目标第20答案

17.解:(1)设等差数列{an}的公差为d(d≠0),等比数列2+d=2{b。}的公比为q,由题意知(2分)2+3d=2q2解得d=2,q=2,(3分)故an=2n-1,bn=2"(5分)(2)当(2n-3)·24n2-12n+12n-1(8分)则T=c1+(2-22+15-3)+(7-5)+…+2242n-1)-2n+1-3+32(n≥2)当n=1时,T1=4也符合上式,2n+所以T=2n+1(12分)

19.解:(1)记A,表示第i道题答对,其中i=1,2,3,4由题意得A1A2A3A4+A1A2A3A4表示得30分的事件(2分)P(AAA,A AAAA)= P(AAAA)+P(A1A2A3A4)=P(A1)P(A2)P(A3)P(A4)+P(A1)P(A2)P(A3)P(A4)=0.7×0.6×0.2×0.9+0.70.4×0.8×0.9=0.2772(5分)(2)X的取值为0,10,20,30,40,50(6分)P(X=0)=0.3×0.4=0.12;P(X=10)=0.7×0.4×0.2+0.3×0.6×0.2×0.1=0.0596;P(X=20)=0.7×0.6X0.2×0.1+0.7×0.4×0.80.1+0.3×0.6×0.8×0.1+0.3×0.6×0.2×0.9=0.0776;P(X=30)=0.2772;P(X=40)=0.7×0.6×0.8×0.1+0.3×0.6×0.80.9=0.1632P(X=50)=0.7×0.6×0.8×0.9=0.3024.(10分)故E(X)=0×0.12+10×0.0596+20×0.0776+300.2772+40×0.1632+50×0.3024=32.112.(12分)

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