考慮數列７，１０，１３，１６，．．．

a
4+3x

2006-12-03 21:16:21 補充：
5(4 3*5) - (4 3*2)= 19 - 10= 9

第n項=3n+4
第５項=3(5)+4
=19
第６項=3(6)+4
=22
第５項與第６項的和
=19+22
=41

a this is+3

b 41

2006-12-24 20:40:19 補充：
a 7+3=10





b 19+22=41


10+3=13

13+3=16

a)3n+4
b)when n=5
3(5)+4
=19
when n=6
3(6)+4
=22
so sum of 5th term and 6th term
=19+22
=41

Let d be the difference of two numbers,
n be the no. of terms

Formula : 7 + (n - 1) d

The 1st difference (d) between 7 & 10:

7 + d = 10
d = 3

Proof: 數列的第 2 項,

7 + (n - 1) d
= 7 + (2 - 1) (3)
= 7 + 3
= 10

Therefore, 數列的第５項,
7 + (n - 1) d
= 7 + (5 - 1) (3)
= 7 + (4) (3)
= 19

Therefore, 數列的第 6 項,
7 + (n - 1) d
= 7 + (6 - 1) (3)
= 7 + (5) (3)
= 22

數列的第５項與第６項的和 = 19 + 22
= 41