Question

高二数学数列的前n项求和1*3,2*4,3*5,...n(n+2)...

Answer 1

an=n(n+1)=n^2+2n

所以Sn=(1^2+2^2+……+n^2)+2*(1+2+……+n)

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

=n(n+1)[(2n+1)/6+1]

=n(n+1)(2n+7)/6