Browsing category ninja maths

The Mathematical Ninja’s Rules of Fractions

The student, wisely, stammered an apology and the Mathematical Ninja pulled him back in through the window by the ankle. "But I just…" "Shht." "I mean…" "Shht." The Mathematical Ninja took a step towards the student and the student, finally shhted1. "Never," said the Mathematical Ninja, "let me catch you

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How the mathematical ninja looks at sums

The sun is setting outside the dojo. The Mathematical Ninja's student knows this, for he is looking out of the window. At some point, he notices the compass point a fraction of a centimetre from his eyeball. "Pay attention," says the Mathematical Ninja. "What does this add up to?" He

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Multiplying by 9 on the fly

A few weeks ago, the Mathematical Ninja explained how to divide by 9 on the fly. This week, it's multiplying that's on his mind - a more common thing to have to do. The multiplying method is easier, too. It's just a case of adding and taking away - and

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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

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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

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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

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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

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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

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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

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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

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