首先要知道1*2+2*3+3*4+……+n(n+1)=1*(1+1)+2*(2+1)+3*(3+1)+.n*(n+1)=1²+1+2²+2+3²+3+.n²+n=(1+2+3+.+n)+(1²+2²+3²+.n²)=n(n+1)/2+n(n+1)(2n+1)/6==n(n+1)[1/2+(2n...
1(n-1)+2(n-2)+3(n-3)+...+(n-1)2+n1=?
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