Science:Math Exam Resources/Courses/MATH312/December 2010/Question 02

MATH312 December 2010

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Question 02
Let $\displaystyle n=3^{100}$ and $\displaystyle a=8585...85$ be the $\displaystyle 2n$ digit number where the digits 85 repeat n times. (a) Is a divisible by 9? (b) Is a divisible by 11? Explain your answers.

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
Remember divisibility rules. A number is divisible by 9 if the sum of its digits is divisible by 9 and it is divisible by 11 if the alternating sum of its digits is divisible by 11.

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. Proceeding as stated in the hint, notice that the sum of the digits of a is $\displaystyle (8+5)n=13\cdot 3^{100}$ and this is clearly divisible by 9. Thus a is divisible by 9. Further, the alternating sum of the digits is $\displaystyle (8-5)n=3\cdot 3^{100}=3^{101}$ and this is clearly not divisible by 11. Thus a is not divisible by 11.

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