SOLUTION: How do you find prime numbers up to 50?

Question 9549: How do you find prime numbers up to 50?
Found 2 solutions by thecampusnerd, rapaljer:
Answer by thecampusnerd(13) (Show Source): You can put this solution on YOUR website!
Remember that prime numbers are numbers that can only be divided by 1 and itself. For example, 1, 3, and 5. The only way you can get 1 is to multiply 1 x 1. The same goes for 3 and 5. 1 x 3 and 1 x 5.

Of course, most people immediately think of odd numbers, but that only holds true until you get to 9. You can get 9 by multiplying 3 x 3, therefore denying it the title of "prime number."

Even numbers can never be prime because they are all always divisible by 2, the only even number that is prime is 2 because the only way to get 2 is to multiply 1 x 2.
Answer by rapaljer(4671) (Show Source): You can put this solution on YOUR website!
Use what we call the Sieve of Eratosthanes, a Greek mathematician who is credited with the method. You list all the integers from 2 to 50 (remember that 1 is NOT a prime number, so you begin with 2!), and you eliminate all the multiples of 2. That is, you eliminate 4, 6, 8, 10, etc.). Next, you eliminate all the multiples of 3. You see, 6, 12, etc are already eliminated, so also eliminate 9, 15, 21, etc. Next eliminate all the multiples of 5 and of 7. What remains will be the prime numbers up to 50. If you want to go higher, you must eliminate multiples of all prime numbers up to the square root of the highest number that you are checking for primes. For example, to check for primes up to 100, you need to check all prime factors up to 10 (i.e., 7). To check for primes up to 200, you need to check all prime factors up to square root of 200, which would be up to 14 (actually 13 will be far enough).

R^2 at SCC
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