(F - 30) / 2 = C
C*2 + 30 = F
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144
Percentanges: e.g. 30% of 70 = 70% of 30 and this is always true. 40% of 60 is 60% of 40 and so on.
Multiplying by 9 with your hands: hold out your hands, palms down. You want to know 1x9, roll up your left pinkie… 9 fingers left so, the answer is 9. 2x9, lower the left ring finger. You have 1 finger, a space, then 8 fingers…answer is 18. 3x9, lower middle finger on left hand. 2 fingers, space, 7 fingers…27. 9x9, lower right ring finger, 8 fingers, space, 1 finger…81. This is not a great concept to explain with text.
Sum of series of numbers: what is the sum of 1 to 10…half way between 1 and 10 is 5. 1+10 is 11. 11x5=55. 1 to 100=(50x(100+1))=5050.
I used to know more but it has been about 15 years since I graduated from uni.
Divisibility tricks :) here are some of the more interesting ones:
7 is probably the most confusing so I’ll add an example. Say you want to check if 161 is divisible by seven.
deleted by creator
It’s not used in computing large primes. We use computers for that and to check divisibility, you just divide and check the remainder.
Well, they do kind of do that, but in a more explicit way. We know that a number is not prime, if it’s a multiple of, well, anything but itself and 1. So, you can for example “sieve out” all even numbers, because all of those are multiples of two. And you can take choose a number that’s a multiple of 3 and then just sieve out all numbers that are that number+3, +6, +9 etc…
The actual sieving techniques are a little more crazy still, but that’s the general idea: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
Most multiplication tables of 1-11 follow a pattern (Excluding 3 and 7). This helps me to know how a number might be divisible by something, or help me remember a multiplication table if I forget it.
Mental calculation of decimal lorgarithms. Ye, not that cool unless you mess with pH.
What do you mean by “reduce” numbers? Divide them by 9?
EDIT: RIP I meant taking the remainder on division by 9
Oh yeah that’s equivalent to taking the remainder on division by 9
I was gonna ask, how often that trick is relevant. I think, the last time I needed to calculate the digit sum was when I was taught in school about the digit sum.
A loosely moderated place to ask open ended questions
If your post is
it’s welcome here!