2倍=1*1+……+1*k+2*1+2*2+……+2*k+……k*1+k*2+……+(k-1)*k-(1²+2²+……+k²)=1*k(k+1)/2+……+k*k(k+1)/2-k(k+1)(2k+1)/6=k²(k+1)²/4-k(k+1)(2k+1)/6=k(k+1)(3k²-4k-2)/12和=k(k+1...