Which sequence is modeled by the graph below? coordinate plane showing the points (1, 1) (2, 2) (3, 4) and (4, 8) a. an = 1(2)n − 1 b. an = 2(1)n − 1 c. an = 1 + (2)n − 1 d. an = (2)(n − 1)

The y-values of 1, 2, 4, 8 imply an exponential function of base 2. At n = 1, y = 1 = 2^0. We can substitute into the various choices to determine the correct equation, which turns out to be a_n = 2^(n-1).