# Use similarity to understand sums with infinitely many terms

A *series* is a sum of infinitely many terms. In some cases
the terms have an understandable pattern and we can calculate the
number that the sum represents. For example, this is one way to
understand repeating decimals like *.11111... = 1/9* or
*.99999.... = 1*.

Using scaling relationships to understand series is a *very*
old idea — at least as old as Archimedes — but is still a cool one. The
Wikipedia article on Geometric series has some other nice pictures to
explain and think about them.

## Fractal patterns

Fractal patterns give intuitive meaning to some other series, and we can use this together with principles of similarity to calculate these series. Here are four fun examples.

### Heart fractal

Since each smaller heart is scaled by a factor of 1/2 from the
previous one, each heart *area* is 1/4 of the previous one.
This means the total red area can be described using an infinite
sequence of additions and subtractions (a *series*) as

1 - 1/4 + 1/16 - 1/64 + 1/256 - ...

But we can also use a scaling relationship between the total region remaining (red) and the total region subtracted (white), and the fact that they combine to form the largest heart shape. This gives two equations that these two values satisfy, and so we can solve to deduce the value of the series!

### Sierpinski triangle

We can use the same idea here: Write an expression for the total area removed as a series (the white area in the picture). Use a scaling relationship between the total white area and the portion in each corner of the large triangle to write an equation that the white area satisfies. Use this do deduce the value of the series! Confirm your answer by recalling the value of a geometric series.

### Tree fractal

In the picture here, each branch is 2/3 the length of the one before it. The total length from the base to a point on the top of the tree is the series formed by adding the lengths of each branch along the way. This forms another geometric series!

### More squares!

This picture shows how to calculate the value of the series1 + 2/4 + 3/16 + 4/64 + 5/256 + 6/1024 + ...

This isn't a *geometric* series, even though it's related to
geometry, because the terms aren't simply powers of a single base
number. But the picture shows a scaling relationship that can still
help us understand its value. If you scale this picture by a factor
of 1/2 and remove the scaled-down part, you will be left with some
area which *is* described by a geometric series! Find the
value of that geometric series, and then use arithmetic to deduce the
value of the original one.

## Warnings!!

The geometry of these pictures makes the related series easy to calculate, but there are other series which are not so easy to work with. The commutative and associative properties don't always hold for values of series (but they do for the ones related to geometry as above)! And not every series you write down has a well-defined numerical value. For example, the Harmonic series is interesting, but does not converge to a specific number.

## Further readings!!

If you're interested in more about the mathematics of series, you could start with the Wikipedia articles on Geometric series and then Convergent series. For some of the more advanced theory, this video by Mathlogger on the Ramanujan identity is thorough and interesting.

tags: fun-topics