Stupid math tricks (idea)

I guess I'm kinda stuck with the "stupid" title, but my trick is far from stupid. This one is really neat when you've done it enough to be able to do it in your head. Here's how it's done.

Have your friend or whomever you're doing the trick for think of a quadratic expression (i.e. 5x²+2x-8). Then, have them give you the value for the expression when x equals 0, 1, and 2. Here's how you can find their expression on paper:

Write the answers in a row (In our example: -8 -1 16). Subtract the first number from the second and the second from the third. Write these two answers in a second row. Then, subtract the second number from the first and write your answer in a third row. You should end up with something like this:

-8   -1   16
   7   17
      10

The coefficient for the quadratic term is half of the last value in your pyramid (5). To get the coefficient of the linear term, subtract half of your last number from the first number in the second row (7-5=2). The constant value is simply the first number in the first row.

Now, you can be all like, "It's 5x²+2x-8, duh". Then they'll be all like, "Dude, you are way smart."

_{Note: This works best on high-school kids.}