通項general term 急急急急急急急急急急

T1 = 2
T2 = 2 + 2
T3 = 2 + 2 + 5
T4 = 2 + 2 + 5 + 8
T5 = 2 + 2 + 5 + 8 + 11
T6 = 2 + 2 + 5 + 8 + 11 + 14
...
Tn
= 2 + 2 + 5 + 8 + ... + [2 + 3(n - 2)]
= 2 + (n - 1)[2 + 2 + 3(n - 2)]/2
= 2 + (n - 1)((3n - 2)/2
= (3n^2 - 5n + 6)/2

The first term T1 = 2
The common ratio r =4/2
=2
Tn=ar^(n-1)
=2(2^(n-1))

2013-10-23 09:53:52 補充：
不好意思,我看錯題目
應該是這樣
n2-n1=d
n4-n3=d+3
...
Tn=a+3(n-1)
=a+3n-3