n(n+1)/2+1/3n（n+1）(n-1)=

1个回答

n(n+1)/2+1/3n（n+1）(n-1)
=(n^2+n)/2+n(n^2-1)/3
=1/2*n^2+1/2n+1/3*n^3-1/3*n
=1/3*n^3+1/2*n^2+1/6*n

1\n(n+1)+1\(n+1)(n+2)+1\(n+2)(n+3)+1\(n+3)(n+4)
1*N+2*（N-1）+3*（N-2）+...+N*1=1/6N(N+1)(N+2)
(n+1)(n+2)/1 +(n+2)(n+3)/1 +(n+3)(n+4)/1
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1(n-1)+2(n-2)+3(n-3)+...+(n-1)2+n1=?
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1\n(n-2)+1\(n-2)(n-4)••••••+1\(n-(n-3))(n-(n-1))=9\20,n=____