Science:Math Exam Resources/Courses/MATH110/December 2016/Question 01 (c)

MATH110 December 2016

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Question 01 (c)
Find the range of $f(x)={\frac {5x+|x|}{x}}$.

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
Find the domain of given function first.

Hint 2
Recall the definition of the absolute value: positive values and zero stay the same, while negative values forget the minus sign and become positive.

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

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 Hint 1, we first find the domain of $f$. Since it is a quotient function, ${\textstyle f(x)}$ is only defined when the denominator doesn't vanish, i.e., ${\textstyle x\neq 0}$. For ${\textstyle x\neq 0}$, we can write $$f(x)=5+{\dfrac {|x|}{x}}.$$ From Hint 2, we recall that ${\textstyle |x|=x}$ when ${\textstyle x\geq 0}$, and ${\textstyle |x|=-x}$ when ${\textstyle x<0}$. This means that ${\textstyle {\dfrac {|x|}{x}}=1}$ when ${\textstyle x>0}$ (note that we exclude the case ${\textstyle x=0}$ here because it is in the denominator) and ${\textstyle {\dfrac {|x|}{x}}=-1}$ when ${\textstyle x<0}$. In other words, ${\textstyle {\dfrac {|x|}{x}}}$ only takes the values ${\textstyle 1}$ and ${\textstyle -1}$. Adding ${\textstyle 5}$ to it, we see that ${\textstyle f(x)}$ takes the values ${\textstyle 5+1=6}$ and ${\textstyle 5-1=4}$. So the range of ${\textstyle f(x)}$ is the set ${\textstyle \{6,4\}}$. Answer: the range of ${\textstyle f(x)}$ is ${\textstyle {\color {blue}\{4,6\}}}$.