Question

1.4.10.20.35.56.84.120.165.220这组数的规律用n表示

Answer 1

an=(1*2+2*3+3*4+...+n(n+1))/2

=(1+2+3+...+n)/2+(1*1+2*2+3*3+...+n*n)/2

=n(n+1)/4+n(n+1)(2n+1)/12