Question 1039348
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1. It was solved before in <A HREF=https://www.algebra.com/algebra/homework/divisibility/Divisibility_and_Prime_Numbers.faq.question.1038874.html>https://www.algebra.com/algebra/homework/divisibility/Divisibility_and_Prime_Numbers.faq.question.1038874.html</A>


2. In order to find that 15! -1 is not divisible by 13, there is no need to calculate this number and then make division.

   Simply 15! contains 13 as a factor, therefore 15! is a multiple of 13.
   Hence, the previous number 15! - 1 is not a multiple of 13.

3. Same logic works for 14! + 1.

4. By answering these questions in <A HREF=https://www.algebra.com/algebra/homework/divisibility/Divisibility_and_Prime_Numbers.faq.question.1038874.html>https://www.algebra.com/algebra/homework/divisibility/Divisibility_and_Prime_Numbers.faq.question.1038874.html</A> 
   I assumed that it it obvious.
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