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yangchuanju 发表于 2021-10-27 13:50 n*(n+1)*(2n+1)=(n^2+n)*(2n+1) =2*n^3+3*n^2+n=3*n^3+3n^2-n^3+n =3*n^3+3*n^2-n*(n^2-1)=3*n^3+3*n^2+ ...
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