Does anyone know some cool math tricks and explain them?

Question

Need to explain them!!!

Answer 1

This trick is mainly fun to do with children 8 to 12 years old.
Ask them how many fingers they have counting the two thumbs. Of course they will answer 10. Tell them no they have 11 and it is mathematically provable.
Tell them to put both hands out palms up. Starting with their left thumb count backwards on their left hand, 10,9,8,7,6, plus the 5 on their right hand equals 11. They can't argue with that.
Try it you will have fun.

Answer 2

one "math trick" that comes to mind is "casting out nines". this is based upon the fact that whenever you transpose two digits in a number (54 instead of 45) the difference between them is always divisible by 9.

"Casting out nines" is an elementary check of a multiplication which makes use of the congruence: 10^x = 1 (mod 9). Let decimal numbers be written
A = An ... A2 A1 A0,
B = Bn ... B2 B1 B0,
and their product be
C = Cn ... C2 C1 C0
Let the sums of the digits of these numbers be A*, B*, and C*. Then
A = A*(mod 9)
B = B* (mod 9)
C = C* (mod 9)
Furthermore, A B = A* B*. So, if C and A* B* are not equal (mod 9), the multiplication has been done incorrectly.

For example, 12345 X 67890 = 838102050. The sum-of-digits of 12345 and 67890 are 15 and 30, respectively, and the product of these is 450. Similarly, the sum-of-digits of 838102050 is 27. And 450 = 27 = 0 (mod 9), so the check shows agreement.

Casting out nines is also an addition test, since A + B = A* + B* (mod 9), and a subtraction test, since A - B = 9 + A* - B* (mod 9). It can also be used as a division test for A / B = Q + R / B(i.e., A = QB + R) since A* = Q* B* + R* (mod 9)

Casting out nines was transmitted to Europe by the Arabs, but was probably developed somewhere on the Indian subcontinent and is therefore sometimes also called "the Hindu check," with "Hindu" simply meaning the people of the Indian subcontinent.

NOTE: the answer above which mentions the "leftmost digit" should really be referencing the right-most digit (least significant digit).

Answer 3

Any whole number multiplied by 9 will eventually equal 9
9x9=81. so 8+1=9

73x9=657 so 6+5+7=18 and 1+8=9

Give it a try.

BTW. Math was my worst subject in school.

Answer 4

Also, to add up all the numbers from 1 to n, the answer is

n(n+1) / 2

For example the sum of 1 to 10 is 55. It's a bit difficult to type an explanation but if you need one it's possible

Answer 5

I hope someone answers because I'd like to know some math tricks too. :-) I'll be watching this question.

Answer 6

0) number is divisible by 2 if leftmost digit is divisible by 2
1) number is divisible by 3 if sum of digits is divisble by 3
2) number is divisible by 2 if leftmost number of 2 digita is divisible by 4
3) divisble by 5 if leftmost digit is 5 or 0

Answer 7

This webpage lists some popular tests for division.

Answer 8

You can use your fingers to know your multiples of 9's