Let a(1) be the first term (which here happens to be 1).
The next term, a(2), is twice the previous term, that is, twice 1, or 2.
a(3) = 2a(2) = 2(2) = 4
This is called a "recursive function."
The 3rd term is 4. The next, which is a(3) is 2*4, or 8.
Note that there's an alternative approach. The first term, a(1), is 1; the next is a(2) = a(1)*2^(2-1) = 1*2^1=2 the next is a(3) = 1*2^(3-1) = 1*2^2 = 4
So the rule is a(n) = a(1)*2^(n-1).
So a(8) is a(1)*2^(8-1) = 1*2^7 = 2^3*2^4 = 8(16) = 128
