绿茶加蜜
幼苗
共回答了199个问题 举报
an=n(n+1),
sn=1*2+2*3+3*4+-----+n(n+1)
=1+2+3+-----+n+(1+2^2+3^2+---+n^2)
=n(n+1)/2+n(n+1)(2n+1)/6
=n(n+1)(n+2)/3
a1=1+1 a2=1/a+4 a3=1/a^2+10 a(n)=1/a^(n-1)+(3n-2)
sn=(1+1/a+1/a^2+…1/a^(n-1)+[1+4+10+…+(3n-2)]
=(a^n-1)/(a^n-a^n-1)+n(3n-1)/2
1年前
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