天雅999
幼苗
共回答了23个问题采纳率:91.3% 举报
an=1+(n-1)*3+(1/2)^(n-1)=3n-2+(1/2)^(n-1)
前10项和S10=1+4+1/2+.+28+(1/2)^9
=(1+4+.28)+[1/2+1/4+(1/2)^9]
=(1+28)*10/2+(1/2)[1-(1/2)^9]/(1-1/2)
=145+1-1/2^9
=146-1/512
=145又511/512
Sn=[1+4+.+(3n-2)]+[1/2+1/4+.+(1/2)^(n-1)]
=[1+(3n-2)]*n/2+(1/2)[1-(1/2)^(n-1)]/(1-1/2)
=n(3n-1)/2+1-(1/2)^(n-1)
1年前
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