Difference in two squares using Quadratic Formula

Here is an interesting one to impress your friends that involves squares and quadratics.

This is quite a simple method for taking away two large squares in your head.

Solve:
10012 - 9992
If you do that on a calculator you'll get 4,000 as your answer and now you ask "how can I do that in my mind"?
Simple, use the quadratic formula.
We know that:
a2 - b2 = (a - b)(a + b)
and so if we apply that to this problem we get...
(1001 - 999)(1001 + 999) = (2)(2000) = 2*2000 = 4000
simple!
Now... lets try something a little Harder (it looks like but with this formula its just as simple!)...

Solve:
10 000 000 0012 - 9 999 999 9992
=(10000000001 - 9999999999)(10000000001 + 9999999999)
= (2)(200000000000)
= 400000000000

Simple.
Enjoy!! :)