求和Sn=（1/a）+（2/a²）+（3/a³）+...+（n/a的n次方）
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Sn=1/a+2/a²+3/a³+...+n/a^n
Sn/a=1/a²+2/a³+...+(n-1)/a^n+n/a^(n+1)
Sn-Sn/a=1/a+1/a²+1/a³+...+1/a^n-n/a^(n+1)
Sn(1-1/a)=1/a*[1-(1/a)^n]/(1-1/a)-n/a^(n+1)
Sn(a-1)/a=1/a*[1-(1/a)^n]/[(a-1)/a]-n/a^(n+1)
Sn(a-1)/a=[1-(1/a)^n]/(a-1)-n/a^(n+1)
Sn=a[1-(1/a)^n]/(a-1)^2-n/a^(n+1)*a/(a-1)
Sn=a/(a-1)^2-1/a^(n+1)/(a-1)^2-n/a^(n+1)*a/(a-1)
Sn=a/(a-1)^2-1/a^(n+1)[1/(a-1)^2+na/(a-1)]
Sn=a/(a-1)^2-1/a^(n+1)[1/(a-1)^2+na(a-1)/(a-1)^2]
Sn=a/(a-1)^2-1/a^(n+1)[1/(a-1)^2+(na^2-na)/(a-1)^2]
Sn=a/(a-1)^2-1/a^(n+1)(na^2-na+1)/(a-1)^2]
Sn/a=1/a²+2/a³+...+(n-1)/a^n+n/a^(n+1)
Sn-Sn/a=1/a+1/a²+1/a³+...+1/a^n-n/a^(n+1)
Sn(1-1/a)=1/a*[1-(1/a)^n]/(1-1/a)-n/a^(n+1)
Sn(a-1)/a=1/a*[1-(1/a)^n]/[(a-1)/a]-n/a^(n+1)
Sn(a-1)/a=[1-(1/a)^n]/(a-1)-n/a^(n+1)
Sn=a[1-(1/a)^n]/(a-1)^2-n/a^(n+1)*a/(a-1)
Sn=a/(a-1)^2-1/a^(n+1)/(a-1)^2-n/a^(n+1)*a/(a-1)
Sn=a/(a-1)^2-1/a^(n+1)[1/(a-1)^2+na/(a-1)]
Sn=a/(a-1)^2-1/a^(n+1)[1/(a-1)^2+na(a-1)/(a-1)^2]
Sn=a/(a-1)^2-1/a^(n+1)[1/(a-1)^2+(na^2-na)/(a-1)^2]
Sn=a/(a-1)^2-1/a^(n+1)(na^2-na+1)/(a-1)^2]
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