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Sn=1*4+2*7+.n(3n+1) =3*1²+1+3*2²+2+…+3*n²+n =3*(1²+2²+…+n²)+(1+2+…+n) =3*n*(n+1)*(2n+1)/6+n*(n+1)/2 =n*(n+1)²
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