My son taught me two Math tricks!

in STEEM CN/中文last year

Yesterday, my son (7-year old) taught me two tricks. I knew one of them.

The first one should be simple, any two digit number times 11 could be done quickly by separating two digits and put the sum of both in the middle.

For example: 23 * 11 = 2 (2+3) 3 = 253
If the sum is bigger than 10 - carry over to the highest.
For example: 56 * 11 = 5 (5 + 6) 6 = (5+1) 1 6 = 616

And here is the second one:
The square of the two-digit number which ends at 5.
For example, 75*75 = 7(7+1)25 which is 5625

The steps are:

  1. Take first digit and multiple its next - resulting the left-most digits of the answer
  2. Append “25” to the above.

Let’s take another example:
65^2 = 6(6+1)25 =4225

I am so proud of my son!

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Great trick! @justyy
I got another. (I don't speak good english so excuse me if there is something I don't say right). The rule is: The sum of odds numbers in sequence starting at 1 is equal to the square of the quantity of numbers you summed.
There is the list for you to check:
So if you want to see how much is 9^2 you just sum nine odds numbers in sequence starting at 1...
9^2=1+3+5+7+9+11+13+15+17(Nine odds nubers in sequence starting at 1).
It matches at any quantity you try.

Thank you very much. It is very interesting! Loving it.