Thread for all Math Geeks...

In this thread you all can post interesting math helper things and ways to find out answers to problems that have been discovered in the past. My favorite one is like this, and was discovered by Pythagoras. I find this most fascinating:

Ever hear of the quick way to multiply 2-digit numbers by eleven?

Provided that the sum of the 2 integers that make up the 2-digit number is less than 10, all you have to do is add them up and stick the sum in between them...

12 * 11 = 132 (1 + 2 = 3, put between the 1 and the 2)

72 * 11 = 792 (7 + 2 = 9, put between the 7 and the 2)

There's such a fantastic mystical character to numbers... gotta love 'em!

Anyone else who has more please post them, these are so fun and interesting. :)

This ones more for the kids but none the less is still usefull for all.
for the nines times tables (only works up to 10 lol)
say u wanted 3 x 9. hold down the 3rd finger on the left hand and count the fingers on the left = 2
and fingers on the right = 7 so its 27
as i said it only works up to 10 times but its a good way to teach your kids.

i just thought of something else based on your aswell
as you said it only works if the sum of the two numbers is less then 10, but with a bit of a twist it can still work
say 88 x 11 = 8 + 8 = 16
add the 1 to 8 and do the rest the same so its 968.
dont know if pythagerous discovered that too. if not give me the credit hehe.

and for 3 numbers say 233 x 11 if u add 2 + 3 = 5 and 3 + 3 = 6 and add first and last numbers around that it seems to work as well 2563
another example 458 x 11 = 4 + 5 = 9, 5 + 8 = 13, add the 1 from the 13 to the 9 which is 10 so you end up with 103 now just put the first and last digits around this exept add the 1 to the 4 so the answer is 5038. starting to get a bit more complex but i think you could go along way with this pattern

Jehnny, that's pretty damn smart! Now I can amaze everyone with my über-leet math skillz... ;) I wish I remembered some good trick like this now... :(
Everyone knows that a^(1/x) is the same as x:th root of a? :P

The number 3025 has a remarkable quirk: if you split its decimal representation in two strings of equal length (30 and 25) and square the sum of the numbers so obtained, you obtain the original number:


(30+25)*(30+25)=3025


The problem is to determine all numbers with this property having a given even number of digits.


For example, 4-digit numbers run from 0000 to 9999. Note that leading zeroes should be taken into account. This means that 0001 which is equal to is a quirksome number of 4 digits.

I make a program in pascal to calculate this numbers..but only with 2,4,6,8 digits..

if you want to see download it.

fantastic. well that is a good tips for make my amaze and may be courage them to love math more since there always stick in their mind, math is very hard subjet and boring. thank jenny

Heres a good math tip. If you are squaring a number that ends in 5, like 25 or 65, then just multiply the tens digit by the next consecutive one, ie for 25 -> 2x3 for 45 -> 4x5 and put a 25 on the end of the answer. It works from 5 to 95 like this.
05 x 05 = (0*1)(stuck onto)25 -> 025
15 x 15 = (1*2)(stuck onto)25 -> 225
25 x 25 = (2*3)(stuck onto)25 -> 625
35 x 35 = (3*4)(stuck onto)25 -> 1225
etc.