Question

15,40,77,126前N项和公式

Answer 1

3*5=15

5*8=40

7*11=77

9*14=126,

an=(2n+1)(2n+3)

=4n^2+8n+3

Sn=4(1^2+...+n^2)+8(1+2+..+n)+3n

1^2+2^2+.+n^2=n(n+1)(2n+1)/6

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

Sn=2n(n+1)(2n+1)/3+4n(n+1)+3n