Science:Math Exam Resources/Courses/MATH220/December 2011/Question 08

MATH220 December 2011

  •  Q1 (a)  •  Q1 (b)  •  Q1 (c)  •  Q1 (d)  •  Q1 (e)  •  Q1 (f)  •  Q2 (a)  •  Q2 (b)  •  Q3  •  Q4  •  Q5 (a)  •  Q5 (b)  •  Q6 (a)  •  Q6 (b)  •  Q7 (a)  •  Q7 (b)  •  Q8  •  

   Other MATH220 Exams

  •  April 2011  •  April 2005  •  December 2011  •  December 2010  •  December 2009  •  

Question 08
Prove that if p is a prime number and p > 4, then $\displaystyle p^{2}\equiv 1{\pmod {6}}$

Make sure you understand the problem fully: What is the question asking you to do? Are there specific conditions or constraints that you should take note of? How will you know if your answer is correct from your work only? Can you rephrase the question in your own words in a way that makes sense to you?

If you are stuck, check the hint below. Consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it!

Hint
Note: this hint was giving on the exam itself. Think about the possible remainders when p is divided by 6.

Checking a solution serves two purposes: helping you if, after having used the hint, you still are stuck on the problem; or if you have solved the problem and would like to check your work. If you are stuck on a problem: Read the solution slowly and as soon as you feel you could finish the problem on your own, hide it and work on the problem. Come back later to the solution if you are stuck or if you want to check your work. If you want to check your work: Don't only focus on the answer, problems are mostly marked for the work you do, make sure you understand all the steps that were required to complete the problem and see if you made mistakes or forgot some aspects. Your goal is to check that your mental process was correct, not only the result.

Solution

Found a typo? Is this solution unclear? Let us know here. Please rate my easiness! It's quick and helps everyone guide their studies. By saying that we look at primes larger than 4, what we really mean is we look at all the primes except 2 and 3 (which makes sense since 22 = 4 and 32 = 9 which is 3 mod 6; so these two don't work anyway). Now any prime p other than 2 and 3 will then clearly NOT be a multiple of two and NOT be a multiple of 3. This means that $\displaystyle p\equiv 1{\text{ or }}5{\pmod {6}}$ since integers that are 0, 2 or 4 mod 6 are all even and since an integer that is 3 mod 6 is a multiple of 3. But since $\displaystyle 1^{2}\equiv 1{\pmod {6}}$ and $\displaystyle 5^{2}=25\equiv 1{\pmod {6}}$ we can conclude that if p is a prime larger than 4, then its square is always 1 mod 6.

Click here for similar questions

MER QGH flag, MER QGQ flag, MER QGS flag, MER RT flag, MER Tag Divisibility, Pages using DynamicPageList3 parser function, Pages using DynamicPageList3 parser tag