23 Comments

1. 

   I confess, I don’t really see how the recipe in panel 2 of the XKCD is meant to lead to the code we were given. That said, there were indeed some formulas I briefly memorized by strategizing something like “You’d think the signs would line up as they do on the LHS, but the trick is to reverse them.” Or what term is numerator and what is denominator. Or what order the functions f and g and h are compounded.

   Yes, there were a bunch of those “not quite what you would ‘naturally’ expect, but a sort of switched-up alignment.” But the house of cards soon falls down when you can’t remember the ones that remain ‘natural’ and those that don’t, or the specific switch-up.

2. 

   Googling derivative of the quotient of two functions

   

   Or, using the link to Wikipedia on “Quotient rule”, https://en.wikipedia.org/wiki/Quotient_rule ,

   In calculus, the quotient rule is a method of finding the derivative of a function that is the ratio of two differentiable functions.[1][2][3] Let
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   {\displaystyle h(x)={\frac {f(x)}{g(x)}}}, where both f and g are differentiable and
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   {\displaystyle g(x)\neq 0.} The quotient rule states that the derivative of h(x) is

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   {\displaystyle h'(x)={\frac {f'(x)g(x)-f(x)g'(x)}{(g(x))^{2}}}.}
   It is provable in many ways by using other derivative rules.

3. 

   Yikes! That didn’t paste at all well!

   It was the same as in the screenshot, but using prime notation rather than dx.

   Anyway, the point was meant to illustrate one of Mitch’s cases. Why a minus sign up top? And which order around it, of the two pairs f’g and fg’?

4. 

   That second excerpt as screenshot

   

5. 

   2 days ago I was reading an article or comic that used the word, “addend”, and I could not remember where it comes up in math.

   I have committed to not looking it up (I should add an until-time to that.) until I remember what it is.

   [10 seconds later]

   Well, I lasted about 10 seconds more and looked it up. It’s a computing term for the number you are adding (i.e. ‘numeric +’) to an existing number. I was thinking it was a math term.

   I just remembered that my Masters work was written in assembly language (for the Zilog Z80). The book for that must mention “addend” a lot.

   I may have never brought that up in a job interview.

6. 

   @kevin, In a/b=c there is a divisor, a dividend, and a quotient. I’m pretty sure that c is the quotient, but have my doubts about the other two. And of course “dividend” has specific meanings in other fields — or indeed, other parts of math.

7. 

   The divisor is the one doing the dividing, at the bottom (-or/-er ending is usually the one doing the thing, actor, writer, etc). The dividend is the thing being divided, at the top (I have a vague idea that the -end part also shows up elsewhere in English, meaning the object of a sentence/action, but can’t come up with an example). How this turned into the equivalent of interest (a fractional addition) in banking/investment…I have no idea. Well, maybe because it is fractional? Still a stretch.

8. 

   I’m fuzzy on whether the term “addend” was introduced in first or second grade arithmetic, but I distinctly remember that in a subtraction problem, the two terms are the minuend and the subtrahend and the result is the remainder. But I don’t remember whether you subtracted the minuend from the subtrahend or the subtrahend from the minuend.

9. 

   Recipe for the code:

   2, 3, 5 and 7 are the first primes. Starting to remember the first primes lead to 2. Then you start to screw up, skip 3, and jump to 5. To screw up even more, you would not only need to skip the next prime and a few more, you would also pick a number that isn’t prime. 18.

10. 

    Thanks, TedD. But I did already get that sequence from the original text. I just didn’t see it as enough. Mostly, it doesn’t give you the 18 in any definite way. So even though it can serve as a story to accompany the number, how can it really be the mnemonic?

11. 

    @jjmcgaffey

    From the Copilot AI engine:

    The term dividend has an interesting etymology. It comes from the Latin word “dividendum,” which means “something to be divided.” In the context of financial and corporate terminology, “dividend” evolved from the practice of dividing a company’s profits among its shareholders

    So, in the context a/b=c the company declares a dividend of $1,000,000 (a) to be divided among 500,000 shares (b), and each share gets $2 (c) as its portion of the dividend.

    This is usually expressed as a dividend of $2 per share, because the relevant amount to the shareholder isn’t the total dividend but the part that they get.

12. 

    The 18 comes from the “increasingly wrong” adding of 1’s to both the zero and the 7 in 07, which is the next prime after 5.

13. 

    Mark H and Mitch – The mnemonic isn’t giving the exact answer, just a way to recall it by effectively storing it in a way that is easy to remember. People are more able to remember “increasingly wrong” and associate that with 18. The last two digits could have been 22 or pretty much any other two “wrong” digits. The association has been made and now recall of “18” (or whatever) is far easier.

14. 

    Well OK. It’s then not a formula for the answer or something to be decoded into the answer, but a “suggester” to put you on the right track — but completing that track is still up to your powers of arbitrary memory.

15. 

    020518 -> if they happen to have been born on February 5, 2018.

    Or the 2nd of May if the way you write dates make any sense….

    G

16. 

    Yes, it’s like wrapping a string around your finger. It reminds you that you have to do something but not what that something was.

17. 

    Sorry it took multiple tries. The system must have been counting you as a first-time-commenter, as your previous contributions had a different name field.

18. 

    My favorite prime number is 57.

    Well, it LOOKS prime.

19. 

    And it’s quite a saucy number!

20. 

    John Conway had a proof that 91 was the first number that looked prime that wasn’t one.

21. 

    I was always known for my memory. Now I am planning to go into my computers and print as a pdf each of my sign ins so I have them recorded. Will have to figure out/how where to store PIN numbers for banks also.

    Since over a year ago when I found out that I have a hidden memory from back in the 1980s (something which happened to me the forgetting of which I consider to be major – husband does not – he was how I found out about this in a casual comment) I have been started to forget the simplest things all the time – as if my mind is working hard to keep me from remembering what husband says is a minor thing – but he only sort of remembers and not the details.

22. 

    A password management tool I have found works quite well for me is Dashlane. With the paid version, it works on all my platforms – windows, iPad, and Android phone. Besides passwords it holds credit card numbers and fills them in when I authorize. What it stores in the cloud is encrypted, and only decrypted on the local platform to insert in your transaction or sign-in.

23. 

    Thank you Mitch.

    I need the passwords to be on a removable drive so I only have to deal with this once and then will have for any of our computers.

    Don’t use credit cards online often at all – we tend not to order things unless we absolutely need it and absolutely cannot find it locally as we always seem to have problems with deliveries. And when we do use a credit card online we use one keep for this purpose only, which has a low balance maximum (at our request).

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