![]() Here is a 100 chart with the multiples of 2, 3, 5, and 7 crossed out. ![]() Perhaps you will want to put a star around it. It turns out that 1 cannot be either prime or composite. If the number 1 were a prime number, we would have to cross out all of its multiples, and that would make 1 be the only prime number!? That would be an unacceptable conclusion. Many teachers have given their students a 100 chart to help them find the twenty-five prime numbers less than 100. Numbers that get crossed out are composite numbers, and they can always be expressed as the product of prime numbers. The next number that has not been crossed out is the next prime number. You can use this method if you make a list of numbers, circle the first available prime number, cross out all of its multiples on the chart and repeat tat procedure over and over again. ![]() Ancient Greek Eratosthenes had a method of finding prime numbers.
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