1 x 5 = 5
5 x 10 = 50 ; 10 is 5+5
50 x 20 = 1000; 20 is 10+10
1000 x 40 = 40,000 ; 40 is 20 + 20
40,000 x 80 = 3,200,000; 80 is 40+40
therefore the next 2 numbers are 40,000 and 3,200,000
2and1
9
12 12
11 and 8
If the first two numbers are 0, 1 or -1 (not both zero) then you get an alternating Fibonacci sequence.
2and1
9
12 12
13, 21 - it is the Fibonacci sequence
The next two numbers in the sequence are... 45 & 54.
-63 and -81
Two consecutive numbers are added together to get the next one. 1,1,2,3,5,8,13...
24, 65.
11 and 8
If the first two numbers are 0, 1 or -1 (not both zero) then you get an alternating Fibonacci sequence.
An arithmetic sequence is a line-up of numbers in which the DIFFERENCE between any two next-door neighbors is always the same.
That'd be the Fibonnaci sequence (this time without the wikipedia URL :P)