## Social Question

# What do you think of this derivation of the formula for geometric series?

The geometric series formula is 1 + x + x^^2…+x^^n = (x^^n+1 – 1)/(x-1). It can be fairly easily derived algebraically, but here is a completely non-algebraic derivation for the case where x is a positive whole number.

Suppose there is a tournament for the game of Monopoly, or whatever multi-player game you prefer. We start with 81 players and there will be 3 players per game. In the first round there are 27 games, and only the winners in each round go on to the next round.

The total number of games is the geometric series 27+9+3+1. Here is another way of getting the total number of games. At the end of the tournament there is a single winner, meaning that 80 players were eliminated. Since two players are eliminated in each game, the total number of games is 80/2 = 40.

This is easily generalized for x players per game and x^^(n+1) players in the first round. The total number of games is the geometric series 1+x+...+x^^n and is also obtained by dividing the x^^(n+1) – 1 players eliminated by the x-1 players eliminated per game.