The Fibonacci sequence is a simple yet extraordinary addition sequence.
1,1,2,3,5,8,13,21,34,55,89,144 and so on
By adding the previous two terms in the sequence, you can find out the next term. For example, 1+1=2, 1+2=3, 2+3=5, 3+5=8 and so on.
The seventh number is 13 and the ninth is 34.