分组求和法,分成一个等差、一个等比: Sn=[(1-1)+(1-2)+(1-3)+...+(1-n)]+(21-1+22-1+23-1+ … +2 n-1) =[-1+(1-n)](n-1)/2+(1-2 n)/(1-2) = -n(n-1)/2 +2 n-1
分组求和法,分成一个等差、一个等比: Sn=[(1-1)+(1-2)+(1-3)+...+(1-n)]+(21-1+22-1+23-1+ … +2 n-1) =[-1+(1-n)](n-1)/2+(1-2 n)/(1-2) = -n(n-1)/2 +2 n-1
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