Question

4+10+18+28+.+n(n+3)

Answer 1

a^2:a的平方

*:乘号

4+10+18+28+.+n(n+3)

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

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

=(1/6)*n(n+1)(2n+1)+3*(1+2+...+n)

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

下面的你自己整理吧