math(sequence) QUICK!!!! 20點

T1 =1
T2 =2
T3 =1+2 =T1+T2=3
T4 =2+3 =T2+T3=5
T5 =3+5 =T3+T4=8
T6 =5+8 =T4+T5=13
....
從以上得知
T(n+2)=T(n)+T(n+1)
SO..
T7 =8+13=T5+T6=21..

2013-04-04 17:57:50 補充：
而點解吾係
Tn= T(n-2)+T(n-1)
因為
當n=1
T(n-2) and T(n-1)是無從得知

1+2 = 3
2+3 = 5
3+5 = 8
5+8 = 13
8+13 = 21

The 7th term is 21
Answer: D.21

T(n+2)=T(n+1)+T(n)
T(7)=T(6)+T(5)=13+8=21

c.20
1+1=2,2+2=3,3+2=5,5+3=8,8+5=13,13+7=20
1,3,5,7,9....