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a=1時,an=n,則Sn=(1+n)*n/2; a!=1時,an=(1-a^n)/(1-a),則Sn=【1-a+1-a^2+…1-a^n】/(1-a)=【n-(a+a^2+…a^n)】/(1-a)=【n-a*(1-a^n)/(1-a)】/(1-a)
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