# Browsing category ninja maths

## The Mathematical Ninja and the First Rule of $\phi$ club

The Mathematical Ninja placed his quadruple espresso on the table; @dragon_dodo looked up from her laptop and smiled. "You're taking it easy on the caffeine this morning, I see?" The Mathematical Ninja nodded. "Yeah, this is only my third cup. What are you working on?" "Computing assignment." The Mathematical Ninja,

## How the Mathematical Ninja approximates factorials, revisited

"That @ColinTheMathmo chap had a blog post on Stirling's approximation, too," said the student, spotting a chance to move the lesson away from his disappointing mock exam results. "Used it to work out 52!" "I saw it," said the Mathematical Ninja, polishing his weaponry smugly. "It... wasn't bad, exactly..." "But

## How the Mathematical Ninja estimates factorials

"I suppose," said the Mathematical Ninja, "I can allow you to put $20!$ into a calculator. There's absolutely no reason you should know that it turns out to be about $2.4 \times 10^{18}$." The student tapped the numbers in, frowned, thought for a moment and said "OK, I'll bite. How...?"

## How the Mathematical Ninja estimates logarithms

"$\ln$", said the student, "of 123,456,789." He sighed, contemplated reaching for a calculator, and thought better of it. "18.4," said the Mathematical Ninja, absent-mindedly. "A bit more. 18.63." The student diligently wrote the number down, the Mathematical Ninja half-heartedly pretended to visit some violence on him, and the student squeaked

## Ask Uncle Colin: two almost-matching sequences

Dear Uncle Colin Somebody told me that the sequences $\left \lfloor \frac {2n}{\ln(2)} \right \rfloor$ and $\left \lceil \frac{2}{2^{\frac 1n}-1} \right \rceil$ were equal up to the 777,451,915,729,368th term, and I shivered in ecstasy. Is there something wrong with me? -- Sequences Considered Harmful When Agreeing Really Zealously Hi, SCHWARZ

## The Mathematical Ninja and the Powers of 10

"So that works out to be $10^{1.6}$," said the student, reaching for the calculator -- and, of course, recoiling as the Mathematical Ninja yelled "yeeha!" and lasso-ed it out of her hand. "Forty," he said. "Too high by half a percent or so. 39.8." The student paused. She would normally

## How the Mathematical Ninja multiplies by 67

A few months ago, @preshtalwalkar at Mind Your Decisions showed off how he'd advise someone to work out $43 \times 67$ using one of my favourite tricks, the difference of two squares. In fact, that's how I'd have approached the question at first, too: the two numbers are 12 either

## The Mathematical Ninja and Ailles’ Rectangle

"$\sin(15º)$," said the GCSE student, and the Mathematical Ninja -- recognising that the qualification recognised idiotic angle measures -- let it slide. "0.2588", he muttered, under his breath, knowing full well that the exact answer -- $\frac{\sqrt{6} - \sqrt{2}}{4}$ -- would get him a blank stare. He sighed the sigh

## Using continued fractions to generate rational approximations

A redditor asks: How would I find a good rational approximation to something like $\log_{10}(7)$? The Mathematical Ninja mutters 0.85 under his breath, as a matter of course, reasoning that $\log_{10}(7) \approx \log_{10}\left(\sqrt{ \frac {10^2 }{2} } \right)$, although my calculator says 0.845098, so he's off by about 0.6%. However,

## How the Mathematical Ninja explains the Mathematical Pirate’s circle trick

"Let me see that!" commanded the Mathematical Ninja, looking at one of the Mathematical Pirate's blog posts. "That's... but that's..." "It's not wrong!" said the Mathematical Pirate, smugly. "It just works!" "But you're presenting it as magic, not as maths." The Mathematical Pirate nodded eagerly. "Lovely magic! How does it