寫 通 項 ( 4 0 分 )

2 , 5 , 8 , 1 1 , 1 4 . . . . . 通項: 3n - 1

1 / 3 , 1 / 9 , 1 / 2 7 , 1 / 8 1 . . . . . . 通項: 1 / 3^n

1 8 , 1 3 , 8 , 3 , - 2 . . . . . .通項: 23 - 5n

2 , 5 , 8 , 1 1 , 1 4 . . . . .
5 - 2 = 3
8 - 5= 3
11 - 8 = 3
14 - 11 = 3
d = 3
a = 2
T(n)
= a + (n - 1)d
= 2 + 3(n - 1)
= 3n - 1

1 / 3 , 1 / 9 , 1 / 2 7 , 1 / 8 1 . . . . . .
3^(-1),3^(-2),3^(-3),3^(-4),....
T(n)
= 3^(-n)
= 1/(3^n)

1 8 , 1 3 , 8 , 3 , - 2 . . . . . .
13 - 18 = -5
8 - 13 = -5
3 - 8 = -5
-2 - 3 = -5
a = 18
d = -5
T(n)
= a + (n - 1)d
= 18 + (n - 1)(-5)
= 23 - 5n

tips:
for arithmetric sequence,
T(n) = a + (n - 1)d
for geometric sequence,
T(n) = aR^(n - 1)
圖片參考：http://hk.yimg.com/i/icon/16/1.gif