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