Science:Math Exam Resources/Courses/MATH312/December 2010/Question 01

MATH312 December 2010

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Question 01
Let n be an integer. Show that there does not exist a prime number $\displaystyle p\geq 2$ which divides both $\displaystyle 8n+3$ and $\displaystyle 5n+2.$

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 hints below. Read the first one and consider it for a while. Does it give you a new idea on how to approach the problem? If so, try it! If after a while you are still stuck, go for the next hint.

Hint 1
Proceed by contradiction. Write out what it means for p to divide the given numbers.

Hint 2
Eliminate for n and factor to reach a contradiction.

Checking a solution serves two purposes: helping you if, after having used all the hints, 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

Proceed by contradiction. Suppose that there exist integers x and y such that $\displaystyle px=8n+3$ and $\displaystyle py=5n+2$. Multiplying the first by 5 and the second equation by 8 gives ${\begin{aligned}5px&=40n+15\\8py&=40n+16\\\end{aligned}}$ subtracting the two equations gives $\displaystyle p(8y-5x)=1$ and as p is prime, it divides the left hand side but not the right hand side which is a contradiction. Hence no such prime number can exist.

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