Did you know?

Percentages are reversible. 8% of 25 is the same as 25% of 8, and often one of them is much easier to do in your head.

@fribbledom % is a mathematical constant equal to 0.01

@ben @fribbledom I sort of knew that but never realized the implications? :blob_dizzy_face:

@fribbledom I've been buried in vector geometry for the past day and this was the best math fact I've heard!

@fribbledom Wtf, never quite really realized it. I must've known but not this obvious. :D

@fribbledom @memethesis :brain1: interpret % as an arithmetic operator
:brain4: const % = 0.01
set mono

@fribbledom 2nd grade knowledge. Why would you even post something so basic?

@LukeAlmighty @fribbledom People can fall out of the habit of using even the most basic techniques, or simply have never noticed them.


Please follow my secondary account @fribbledom_advanced_math where we discuss the Standard Model and the Callan-Symanzik equation 😄

On a more serious note: yes it's obvious if you think about it, but just look at all the replies and you'll realize why I post it.

@LukeAlmighty Wow mate I can't believe some people actually didn't realize this.... like lol what idiots dude, these are the exact types of people that fall for email phishing attacks and lose access to their WoW accounts like the kids they are.

@LukeAlmighty @fribbledom does it really make you feel bigger to shame people who don't know a math thing? sounds like you missed a much more important lesson than this in second grade

@relsqui @fribbledom
Wow... I have seen many things before. I understand there are people who get a very poor education (and one toot about multiplication won't solve that), but to see how many people in here are proud of having a miserable education is truly staggering.

But be proud of whatever you want. As long as it doesn't affect your life, I don't care either.

@LukeAlmighty @fribbledom my education was fine. I'm proud of my ability to be kind when someone else doesn't know something I know.

@relsqui @fribbledom Strange. What you postad was prejudiced at best. That is the exact oposite of kindness.

But if you just want to get angry since I stated there is a problem, you will only lead a miserable life. And that helps noone. Does it?

@LukeAlmighty @fribbledom I didn't learn percentages until fifth grade, nor the algebra required to prove the conjecture until 7th grade; so, there's that...

@fribbledom I've got one which I found out when drawing geometric shapes one day:

If you count up to a number from 1 and down again

1, 2, 3, 4, 5, 6, 7, 6, 5, 4, 3, 2, 1

Then all the numbers add up to the central number².

@BongoBaggins @fribbledom

break it down into a sequence up to middle number:

Tn = n; a = 1
:. Sn = n/2(2a + d(n-1)) = n/2(2 + n - 1) = n + n²/2 - n/2 = n² + n/2

Second one is second half minus middle :. Sn2 = Sn - Tn = n²/2n + n/2 - n = n²/2n - n/2

Adding, we get:
n²/2 + n/2 + n²/2 - n/2 = n²

@BongoBaggins @fribbledom haha ☺

We were just doing this in class. For arithmetic sequences (where the difference between terms is constant), term n = a + d(n-1) where a is first term and d is difference, and sum to n terms is n/2(2a + d(n-1)). So I just applied those. It's fresh cos we just wrote a test on it haha

@BongoBaggins @fribbledom How does that sums up to the square value? it seems to work for numbers I tested but.. why

@BongoBaggins @fribbledom I get it

so for y=x+1


so each time you want to go from x² to y², you need to add x+y to x²

@fribbledom so for that example, the answer is 2, right? I'm reallyvbad a maths, so I really want to be extra sure I understood correctly

@fribbledom then, why the frick didn't they teach us that at school? This would have saved me years of bother!!

Thank you for sharing. You have no idea how much this helps!

@fribbledom I have many such tricks. A bunch I use rely on 1/(1+x) ~= 1-x, when x is small (specifically, when x*x can be neglected). So, 18/19 = 18/(18+1) = 1/(1+18) ~= 1-1/18. 1/18 is close to 1/20 or .05. So 18/19 is about .95 (actually .947), and I did it entirely in my head without any actual division! (If you use this trick to approximate 1/18 = 1/(20-2)=1/(20*(1-2/20))=1/(20*(1-1/10)) ~= 1/20 * (1+1/10) = .055, you end up with .945. Again, these are steps one can do in one's head.)

@fribbledom Spot the typo. :) That first bit after 18/(18+1) should be 1/(1+1/18) not 1/(1+18).

@fribbledom !!
That absolutely makes sense and I had absolutely never thought of that.

@funnypanja @fribbledom no. 2 is 8% of 25 because of my decision. Everything else was my stupid neighbors to blame.

@fribbledom I remembered that there was a trick like that, but for the last few days I was trying to remember Perfect timing thanks! (note that I didn't actually look into it, I was just lightly thinking about it)

@fribbledom you mean a*b=b*a or (a/100)*b=a*b/100 .. it sounds very boring stated that way.


I think this makes it a bit more readable:

a% of b = (a/100) * b = 0.01 * a * b


b% of a = (b/100) * a = 0.01 * b * a

@fribbledom @jasper

so 25 * 8 = 200 and 8 * 25 = 200, and 200 * 0.01 = 2.


25 percent of 8 is 2


8 percent of 25 is 2


Exactly. I don't know why it would be news that 8*5 is the same as 5*8 and just move the decimal place to make sense. People do this all day when out shopping don't they.

@fribbledom did u know? multiplication in the rational numbers is commutative, so 25/100 * 8 = 25 * 1/100 * 8 = 25 * 8 * 1/100 = 25 * 8/100 = 8/100 * 25.

@fribbledom also, it's important that the multiplication is associative, otherwise (25 * 1/100) * 8 = 25 * (1/100 * 8) may not be true.

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