A fast little trick for mental calculation of x^2

I suppose many might have figured this out, but it's a neat little mind trick for calculating the product of a number between 10 - 19 with itself. I stumbled upon it as I was riding the bus back home from my math class, and thought it might come in handy for those who don't know it yet!

Multiplication:

Think:

Excellent, should take a normally gifted person about seconds! ;)

For numbers which gives two digits for any of the operations, you do the same basically. But just remember that you need to summarize the results you get correctly.

Multiplication:

Think:

If you remember the addition you need to do, it shouldn't take much longer.

This metod works excellently for myself, and it will usually take me between 2-5 seconds to calculate the product of any number between 10-19 multiplicated with itself.

A bit more mathematically we can write:

Where alpha denotes the first digit of the product, and beta the second.