How do you write a exponential growth function

Consider for a moment building a tree with all possible choices. The first row would contain 8 books (each possible choice for the first book). Then, the second row would contain 7 books for each book in the first row (the choices possible for the second book). Otherwise, it would be 8*7. Following this principal, we continue to multiply down to 1. 8*7*6*5*4*3*2*1. This is also known as 8!, a factorial.
Finally, 8! = 40,320