Which sequence is modeled by the graph below? coordinate plane showing the points 1, 2; 2, 4; 3, 8

First, we inspect what type of sequence is the order of the coordinates:
a2 = 1
a3 = 2
a4 = 4

Getting the difference,
a3 - a2 = 1
a4 - a3 = 2

The differences are not equal; hence, the sequence is not arithmetic.

Getting the ratio:
a3/a2 = 2
a4/a3 = 2

The common ratio is 2. Using the general form for a geometric series:
an = a1 r^(n-1)
If n = 2
1 = a1 (2)^(2-1)
a1 = 1/2

So,
an = (1/2) (2)^(n-1)