Cool Math Trick , Mind Reading Trick

The following math trick always gives back the number (whole number) you thought of.

For example, choose 6:
1+2+3+4+5+6=21
Then multiply the result by 2: 21*2=42
Then we subtract (your number)^2 from 42: 42-6^2=6 which is the number we originally chose

Note: "..." means continue the sum until you reach the last number. For example: 1+2+...+6=1+2+3+4+5+6

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[Hiromichikanasugi]

somethings wrong with the example i think.. becoz u said(think of a number. add 1+2..+(yournumber)) so supposedly its should be.. 1+2+6? not 1+2+3.. coz 6 is your number.. maybe you need to rephrase it.

[dmathguy]

@Hiromichikanasugi

the three dots (...) mean that you have to continue the sum until you reach your number, for example:

1+...+3=1+2+3=6

1+...+6=1+2+3+4+5+6=42

1+2+...+6=1+2+3+4+5+6=42

[998Nikki]

what is the, "..." ?

[dmathguy]

@998Nikki

"..." means continue the sum until you reach the last number.

[dmathguy]

If you don't understand it, please see the example.

[Hfrimanhfriman]

Hi, what do you mean by

"add 1+2+. . .+yournumber"

because i am getting something wrong.

My number = 6

1+2+3+4+5+6 times 2 =

42 is not a square number!

:(

[dmathguy]

For example, choose 6:

1+2+...+6=21

Then multiply by 2: 21*2=42

Then we subtract 6^2 from 42: 42-36=6 which is the number we originally chose

[Crumbsm8]

1+2+3+4+...+n

the sum from r to n starting at r=1 is (n(n+1))/2

multiply by 2 gives n(n+1)=n^2 +n

subtracting n^2 gives n

It's not possible to get this wrong once you know the maths behind it.

[TheJessi131310]

didnt work i thought bout 5 and the end was 36!

[foodforthoughtFEAST]

@peachthehamster1 What number did you use? 3?

[MrSquiggle1000]

16.5

[peachthehamster1]

it worked for me!

[1812jacko]

@Alysha2001 no shit ... +10 then +20 = +30... So what you just said effectively was:

Think of a number

+30

Take your number

=30