Science:Math Exam Resources/Courses/MATH105/April 2012/Question 01 (b)

MATH105 April 2012

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

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Question 01 (b)
You are given the formula for the sum of a geometric series, namely: $1+r+r^{2}+\cdots ={\frac {1}{1-r}},\qquad |r|<1.$ Use this fact to evaluate the series in part (a).

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 that $1+r+r^{2}+\dots =\sum _{k=0}^{\infty }r^{k}$ Try to find a suitable value for r in our case.

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

The trick here is to recognize the series given as a geometric series. The question provides the formula $1+r+r^{2}+...=\sum _{k=0}^{\infty }r^{k}={\frac {1}{1-r}}$ for $|r|<1.$ Essentially, would like to evaluate the series by finding a suitable r so that this formula can be applied. Let's first re-express $\sum _{k=0}^{\infty }(-1)^{k}2^{k+1}x^{k}=\sum _{k=0}^{\infty }2(-1)^{k}2^{k}x^{k}$ as $2\sum _{k=0}^{\infty }(-1)^{k}2^{k}x^{k}$ so that all terms in the summation have an exponent of k and the indexing starts at 0. Now we note that $2\sum _{k=0}^{\infty }(-1)^{k}2^{k}x^{k}=2\sum _{k=0}^{\infty }[(-1)(2)(x)]^{k}=2\sum _{k=0}^{\infty }(-2x)^{k}$. The formula provided now works with $r=-2x$. The series sums to $2\times {\frac {1}{1-(-2x)}}={\frac {2}{1+2x}}.$

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