A finite geometric series is the sum of a sequence of numbers. Take the sequence 1, 2, 4, 8, … , for example. Notice that each number is twice the value of the previous number. So, a number in the sequence can be represented by the function f(n) = 2n–1. One way to write the sum of the sequence through the 5th number in the sequence is ∑5n-12n-1. This equation can also be written as S5 = 20 + 21 + 22 + 23 + 24. If we multiply this equation by 2, the equation becomes 2(S5) = 21 + 22 + 23 + 24 + 25. What happens if you subtract the two equations and solve for S5? Can you use this information to come up with a way to find any geometric series Sn in the form ∑an-1bn-1?