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23 29.
They are 23 and 29.
211 is the only prime number between 200 and 220
List all the factors of each number. Which factors are prime factors between 20 and 28.
Euclid (c. 300 BC) was one of the first to prove that there are infinitely many prime numbers. His proof was essentially to assume that there were a finite number of prime numbers, and arrive at a contradiction. Thus, there must be infinitely many prime numbers. Specifically, he supposed that if there were a finite number of prime numbers, then if one were to multiply all those prime numbers together and add 1, it would result in a number that was not divisible by any of the (finite number of) prime numbers, thus would itself be a prime number larger than the largest prime number in the assumed list - a contradiction.