How to Find if a Number is Prime or Not (Strategically)

Hello there! It's been a while since I did a video about prime numbers but today I want to show you how to best utilize your time when you are faced with a problem where you need to find out if the number is prime or not. Imagine a scenario where you don't have a calculator or google. Sometime we have really big numbers and we don't know where to even begin. Testing a bunch of random numbers isn't exactly the best method so we can try to estimate what the top limit of numbers we need to test up to is.

What you're going to do is find the square root of the number or the closest whole number and test the prime numbers until that number. For example, let's take a look at 81. Imagine we didn't know if 81 was a prime number or not. The square root of 81 is 9 so all we need to do is test all the prime numbers up to 9, which are 2, 3, 5, 7. From these tests, we will find that 81 is actually divisible by 3 so it is not a prime number. In this video, I show the steps on finding the "upper limit" for prime numbers to test on two smaller numbers. I will then go over 3 different "bigger numbers" that are 3 digits. For the purposes of this video, I won't use HUGE numbers but you can definitely apply this to the bigger numbers.

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