# Browsing category ninja maths

## Summing a geometric series: Secrets of the Mathematical Ninja

"I don't know if you were actually working stuff out there, or if you just muttered at random for a bit and guessed," said the Mathematical Ninja's cheeky student. Stung, the Mathematical Ninja was forced to explain how he figured out $\left(1 - \frac{1}{8}\right)^{19}$. It's a simple enough trick that

## Squaring biggish numbers, revisited

One of the Mathematical Ninja's favourite tricks is squaring biggish numbers. He'd secretly like to be Art Benjamin one day. So (inspired partly by Barney), he's been looking at quick tricks to help him square numbers. He knows the first 25 square numbers by heart (and so should you); he's

## Why the Mathematical Ninja hopes for a hard exam

The Mathematical Ninja sighs. "Say that again?" "I hope it's an easy paper!" says the student, brightly. Wearily, the Mathematical Ninja goes to the board and sketches two curves. They look a little like boa constrictors that have swallowed elephants. "You're above average, correct?" he asks. The student doesn't look

## Dividing by 9 – Secrets of the Mathematical Ninja

Dividing by 9 has always been an awkward one for the mathematical ninja - it ought to be a simple operation, but for some reason it's never stuck. However, there IS an easy way that involves not much more than adding up and (possibly) using your fingers to track a

## The Rule of 72: Secrets of the Mathematical Ninja

Despite what you may have heard, Einstein probably never said that compound interest was the greatest force in the universe. It is, however, an interesting beastie. The Mathematical Ninja likes quick fixes. The Mathematical Ninja LOVES estimating powers of e, but he loves quick-and-dirty estimates even more. Especially to one

## Remembering how to differentiate – Secrets of the Mathematical Ninja

The Mathematical Ninja makes ample use of mnemonics to remember how to do just about everything. He was quite upset when Pluto was demoted, because "My Very Easy Mnemonic Just Serves Up Nine" doesn't make any sense. In particular, the Mathematical Ninja has a creed. A strict set of rules

## What day of the week is it? It’s DOOMSDAY! Secrets of the Mathematical Ninja

Today's the day you should be outside looking for the man with as many noses are there are days left in the year! However, if you've spotted him, you could turn your attention to something all mathematicians have trouble with: knowing what day of the week it is. This is

## Recurring Decimals – Secrets of the Mathematical Ninja

— Thanks to Rosalind for showing me this trick. It’s one of the questions in the GCSE that looks like it ought to be easy: What is $0.1\dot{4}3\dot{6}$ as a fraction? But it’s a lot less easy than it seems at first. I’ve taught the longwinded way for years. It

## Thirteenths (Part 3/3): Secrets of the Mathematical Ninja

The final installment, the big reveal: why does the ninja trick of multiplying by 77 and finding the nine’s complement work? My friend, that is an excellent question. The reason is this: $77 \times 13 = 1,001$. And it turns out, 1001ths are not all that hard to work out.

## Thirteenths (Part 2/3): Secrets of the Mathematical Ninja

This is the second of a three-part series about working out thirteenths. In the first part, you learned that the first step of finding thirteenths was to multiply by 77. The second part is to work out the nine’s complement of one less than your number. What’s the nine’s complement?