1510 Prime Time Numbers

When it comes to prime numbers I ask myself, who comes up with these numbers? At first look, they are just numbers, but what really makes them prime numbers? By definition, a prime number is one that can only be divided by one, and itself, everything else is a composite number. How did mathematicians figure this out? Is there a pattern?

Come to find out, there is a pattern! A Greek man named Eratasthenes, studied mathematics over 2000 years ago came up with a model that is called the Sieve of Eratosthenes. Essentially, it takes the first prime number, 2, and then crossed off every 2nd number from there on out. The next prime number is 3 so, he crossed out every 3rd number and continued that pattern with each prime number. All the numbers he was crossing out were composite numbers. He was making a sieve that drains out composite numbers and leaves prime numbers behind. Is this not crazy? I'm just amazed on how this works.

The question you always hear from a student in class is, why are prime numbers important? Is there any use for them outside of math? You bet there is a use for them and I bet you didn't even know it. Large prime numbers are used in RSA cryptographs for sending secret information like your credit card over the internet. The first step is to take 2  large prime numbers, multiply them together and their product is what is what makes factoring out the prime number difficult. Watch the video and try and follow along, it's pretty complicated after the first step I just explained but well worth the view.

1510 Divisible Numbers

Who says you can't teach an old dog new tricks? With the amount of math classes I've taken over the years, I've never been taught some of the shortcuts that are out there for testing number divisibility. I was blown away by how easy this process is. Any textbook out there will refer to them as rules of divisibility, but I still like to call them shortcuts because of how easy it just made my life...too bad it took 30+ years.


The "rules" state, a number is divisible when you can divide one number by another and the result is a whole number. Now, for every number 1-12, there is a special rule that follows it and I encourage you to take a look at these tricks. It's almost freaky how this works out and it makes you wonder how someone figured this out. Knowing these tricks, rules, or shortcuts can really make reducing numbers or fractions a breeze. I can't tell you how long I would spend punching each number into the calculator to see if it was divisible or not. Now, I've made a little cheat sheet with all the rules on it that I keep with my math work and just refer to it when I need it.

Students will be able to memorize 2, 5, and 10 with no problems because they are easily recognizable, but what about 3 or 9? Here is a video that might help them out.

I aspire to be this teacher in a couple of years. If I can rap, I can get my students to learn the divisibility rules, or shortcuts!