**Proof is left as an exercise for the reader.**

## 1. If you write out pi to two decimal places, backwards it spells “pie”.

BuzzFeed / Kelly Oakes / Via Twitter: @TrueFacts

3.14 = PIE.

## 2. A French word for pie chart is “camembert”.

Ezergil

Because of course it is.

## 3. The spiral shapes of sunflowers follow a Fibonacci sequence.

Irantzu_Arbaizagoitia

That’s where you add the two preceding numbers in the sequence to give you the next one. So it starts 1, 1, 2, 3, 5, 8, 13, 21, etc. The Fibonacci sequence shows up in nature a fair bit.

## 4. The Fibonacci sequence is encoded in the number 1/89.

Jupiterimages / Thinkstock

1/89 = 0.01 + 0.001 + 0.0002 + 0.00003 + 0.000005 + 0.0000008 + 0.00000013 + 0.000000021 + 0.0000000034 etc.

## 5. A pizza that has radius “z” and height “a” has volume Pi × z × z × a.

Because the area of a circle is Pi multiplied by the radius squared (which can be written out as Pi × z × z). Then you multiply by the height to get the total volume.

## 6. The word hundred is derived from the word “hundrath”, which actually means 120 and not 100.

mshch

Hundrath is Old Norse.

## 7. 111,111,111 × 111,111,111 = 12,345,678,987,654,321.

Via studentbeans.com

It also works for smaller numbers: 111 × 111 = 12321.

## 8. In a room of just 23 people there’s a 50% chance that two people have the same birthday.

DimitrovoPhtography/DimitrovoPhtography

It’s called the Birthday Problem. In a room of 75 there’s a 99% chance of two people matching.

## 9. Zero is the only number that can’t be represented in Roman numerals.

Flickr: wwarby / Creative Commons

The Latin word “nulla” would have been used instead.

## 10. (6 × 9) + (6 + 9) = 69.

## 11. We tend to think of odd numbers as male and even numbers as female.

crazydiva

This ancient belief was tested by James Wilkie and Galen Bodenhausen of Northwestern University. In his latest book, Alex Bellos writes: “They showed respondents randomly assigned pictures of the faces of young babies, each next to a three-digit number that was either odd-odd-odd or even-even-even, and asked them to guess the baby’s sex […] Respondents were about 10 per cent more likely to say that a baby paired with odd numbers was a boy, than if the same baby was paired with even numbers.”

## 12. If you shuffle a pack of cards properly, chances are that exact order has never been seen before in the whole history of the universe.

Via giphy.com

## 13. Zero is an even number.

daizuoxin / Thinkstock

But people take longer to decide whether it’s even or odd because it’s not as easy for us to mentally categorise.

## 14. There’s not enough space in the known universe to write out a googolplex on paper.

According to Carl Sagan in the original Cosmos series. A googolplex is 10 to the power of a googol, or 10 to the power of 10 to the power of 100. This website will write it out for you (or start… it won’t ever finish because your computer won’t have enough memory).

## 15. The most popular favourite number is 7.

Flickr: losmininos / Creative Commons

Nearly 3000 people, around 10% of the total asked, chose 7 as their favourite number in an online poll by Alex Bellos. The second most popular was 3.

## 16. That might be because 7 is “arithmetically unique”.

Flickr: pagedooley / Creative Commons

It’s the only number below 10 you can’t multiply or divide and keep within group. For example, 5 you can multiply by 2 to get 10 (still within the 1-10 group), 6 and 8 you can divide by 2.

## 17. 7 also shows up a lot in human culture.

We have seven deadly sins, and seven wonders of the world. Not to mention colours of the rainbow, pillars of wisdom, seas, dwarves, days in the week…

This might be because when these things came about there were celestial bodies visible in the sky (the Sun, the Moon, Venus, Mercury, Mars, Jupiter and Saturn).

## 18. The number 4 is considered unlucky in much of Asia.

Flickr: boklm / Creative Commons

That’s because the words for “four” in Japanese, Cantonese, Mandarin and Korean (shi, sei, si, sa) sound the same as the words in those languages for death.

## 19. .999999… = 1

Flickr: andrec / Creative Commons

Here’s the proof:

If 10N = 9.9999…Then N = .9999….Subtract N from 10N, leaving you with 9N=9.So then N=1. But we already know that N=.9999… as well. So 1=.9999….

## 20. Cicadas use prime numbers as an evolutionary strategy.

Flickr: oakleyoriginals / Creative Commons

Cicadas incubate underground for long periods of time – 13 or 17 years – before coming out to mate. 13 and 17 are both prime numbers. It’s thought cicadas ended up in these prime number life-cycles because it meant they came into contact with predators on more round numbered life-cycles less often.

## 21. 10! seconds is exactly 6 weeks.

agafapaperiapunta/agafapaperiapunta

10! means 10 factorial. 10! = 10 × 9 × 8 × 7 × 6 × 5 × 4 × 3 × 2 × 1 = 3628800 seconds. Which is 42 days, or 6 weeks, exactly.

## 22. Take any four digit number, follow these steps, and you’ll end up with 6174.

stevanovicigor / BuzzFeed

1. Choose a four digit number (the only condition is that it has at least two different digits).2. Arrange the digits of the four digit number in descending then ascending order. 3. Subtract the smaller number from the bigger one. 4. Repeat.

Eventually you’ll end up at 6174, which is known as Kaprekar’s constant. If you then repeat the process you’ll just keep getting 6174 over and over again.

## 23. 555 is used by some in Thailand as slang for “hahaha”, because the word for “five” is pronounced “ha”.

## Thanks to this Reddit thread, this Quora post and Alex Bellos’ new book, Alex Through The Looking Glass for some of these facts.

Read more: http://buzzfeed.com/kellyoakes/maths-facts-youll-probably-never-need-to-use