Prime Numbers

Is there any real good formula(s) for finding out if a number is prime? I've managed to devise a formula on my own, but it doesn't work when you plug in something that has a multiple of three. The formula would yield either a whole or a fraction. A fraction means that the number you plugged in is a composite number, whereas a whole number meant you got yourself a genuine prime number. But, when I plug in a multiple of 3, I get a whole number and that's no good.

So, if anyone has a favorite formula or if there's just one good formula right now, please do share. Thanks.

http://img228.imageshack.us/my.php?image=primeformulahb4.jpg

n would be the same number as c.

http://img156.imageshack.us/my.php?image=primeformulasimpletestyo8.jpg

This is a simple test of it. I've applied a sleuth of numbers, but it never works right with multiples of 3. As you can see, the problem produces a whole number and according to my theory, that makes c a prime number.

Pardon how ugly it looks. I don't have anything to write out formulas like that. I also have a pretty minimal understanding of Calculus.