sequence

2+5,2+5+8,2+5+8+11,. .....
a1=2
a2=5
a3=8
an=2+3(n-1)=3n-1
Sn=a1+a2+...+an
Sn
=Σ(3k-1) [k from 1 to n+1]
=3Σk-(n+1)
=3(n+1)(n+2)/2-(n+1)
=(n+1)([3(n+2)/2]-1)
=(n+1)(3n+4)/2


2006-11-29 22:52:29 補充：
Sn=a1 a2 ... an 1