# October, 2016

## On the trick with 9s

Pick a number, any number1 Let's say 24,876,028, one of my favourites in the 20-millions. I can tell by looking at it that it's one more than a multiple of 9. It's not that clever a trick: I just added up the digits (getting 37), and repeated the trick with

## Ask Uncle Colin: how to do moments questions?

Dear Uncle Colin, I'm struggling to make sense of questions about moments in M1. Can you help? -- Failing Equilibrium-Related Moments... All Torque! Hi, FERMAT! That's a bit more general than the kind of question I usually answer, but no matter! Here’s the briefest theory of equilibrium I can come

## The Great Straight Lines debate

* Edited 2016-10-24 to add categories, just for Cav.

## Ask Uncle Colin: Am I working too hard?

Dear Uncle Colin, I work in a call centre, which is run by the numbers. Every employee has to stay above a certain conversion rate, calculated as the number of successful deals resulting from their calls, divided by the number of times they have a call that doesn't go to

## The Mathematical Ninja And The Cubes

The Mathematical Ninja glanced at the Rubik Cube and paused. "And $45^3$ is..." A reach for the calculator. A flurry of colour. "Ow!" "91,125," said the Mathematical Ninja, catching the cube on the rebound and swizzling it solved. "Only needed 12 that time." The student sighed. "Go on, then. I

## Ask Uncle Colin: A Huge Power Of Two

Dear Uncle Colin, I've been asked to find $2^{64}$ without a calculator, to four significant figures. How would you go about this? -- Large Exponent, Horrific Multiplication, Extremely Repetitive Hi, LEHMER! To get a rough answer, I'd usually start with the rule of thumb that $2^{10} \approx 10^3$. I'd conclude

## Wrong, But Useful: Episode 37

In the October episode of Wrong, But Useful, @icecolbeveridge and @reflectivemaths have their habitual chat. Dave insults mechanics, then gets stroppy about Colin insulting stats The number of the podcast is 16, 40 and 64, because one number isn't enough for Dave Dave visits a fete and gets ripped off

## Ask Uncle Colin: Invariant Lines

Dear Uncle Colin, I've got a matrix, and I'm not afraid to use it. It's $\begin{pmatrix} 3 & -5 \\ -4 & 2\end{pmatrix}$ Apparently, it has invariant lines. Those, I'm afraid of. How do I find them? -- Terrors About Rank, Safely Knowing Inverses Hi, TARSKI! An invariant line of

## A curve-sketching challenge

Via @DrTrapezio, an interesting question: Year 13 curve sketching challenge: y = | | | |x| - 1| - 1| - 1| — Luciano Rila (@DrTrapezio) July 14, 2016 Where do you even start with that mess? The answer to that, my friend, is you start in the middle and

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##### Where do you teach?

I teach in my home in Abbotsbury Road, Weymouth.

It's a 15-minute walk from Weymouth station, and it's on bus routes 3, 8 and X53. On-road parking is available nearby.