## Proof Without Words: Two Dimensional Geometric Series

I offer this visual proof of the following amazing infinite sum

$\frac{1}{4} + \frac{2}{8} + \frac{3}{16} + \frac{4}{32} + \frac{5}{64} + ... = 1$

1. Graeme McRae says:

Now, let’s see a similar 3-dimensional proof that 1/8 + 3/16 + 6/32 + 10/64 + 15/128 + … = 1

• MrHonner says:

I will leave that as an exercise for the reader.

• Mike Lawler says:

Here’s two attempts at the 3d visual proof that 1/8 + 3/16 + 6/32 + 10/64 + . . . = 1 using Lego Digital Design. It was neat to see the cube coming together while building this. The first video is the first three terms, and the 2nd is the first 5.

2. Nat Silver says:

JIm Loy sums the series on his website, using partitions:
http://www.jimloy.com/algebra/aseries.htm A standard approach would be
S = (1/4)(1+2x+3x^2+4x^3+…) where x = 1/2.
S = (1/4)(d/dx(x+x^2+x^3+…))
= (1/4)(d/dx(x/(1-x)))
= (1/4)(1/(1-x)^2) where x = 1/2,
as I’m sure you are aware.