Mental Math Tricks

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Squaring numbers

Take any number x. If it is not easy to calculate x2 in your head, then use this formula: x2 = (x+d)*(x-d) + d2 where d is the difference between x and the nearest multiple of 10.

Squaring example

So if you wanted to calculate 232 you can use the formula above to do 20*26+32 = 520+9 = 529.

Formula Explanation

If you were to expand (x+d)*(x-d) you would get x2 + x*d - x*d -d*d = x2 - d*d. Since negative d squared is left after the other ds get cancelled we must add d squared to cancel out negative d squared.

Convert repeating decimals to fractions

Take any repeated decimal and take one full cycle of digits before it repeats and divide by the same number of 9s.


.51515151… = 51/99.


coming soon…

Date: 2014-06-28T05:01+0000

Author: Korey Hinton

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