Try different numbers until you find one that is prime. A simple way (but not necessarily the fastest way) to check whether a candidate number is a Prime number is to check whether it is divisible by any number smaller than the number itself (and excluding 1). To make this somewhat faster, divide by 2, and then by all odd numbers, up to the square root of the number. For example, for numbers up to 100, you only need to test divisibility by 2, 3, 5, and 7. 9 is not a prime number, so you don't need to divide by 9; although according to the simple rule explained above, you would divide by 9, too. And 11 is greater than the square root of any number less than 100.
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The way to know the Prime Numbers is to find out if they can only Multiplied Once ,The way to know Composite Numbers is to find out if they can be Multiplied Twice
Eratosthenes lived between 276 and 194 B.C. He didn't discover prime numbers; he devised a simple way to determine what numbers are prime in a given range.
There is no simple way. The difficulty wit prime numbers is that there is no pattern.
do the prime factorization of the 3 numbers. list the prime factors of all the 3 numbers. circle the factors that are common to the 3. multiply them. that number is the HCF
The same way as with smaller numbers, it may take longer. Just keep dividing by prime numbers until all the factors are prime.