Problem:
Show that there are infinitely many prime numbers whose last digit is not .
Solution
Solution:
Assume there are only finitely many such primes . Consider the number
Since has last digit , there must be a prime dividing which does not have last digit (otherwise must have last digit ). But by construction, cannot divide , because it leaves a remainder of when divided by . This is a contradiction, so our assumption was wrong and there must be infinitely many such primes.
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