Science:Math Exam Resources/Courses/MATH 180/December 2017/Question 05 (a)

MATH 180 December 2017

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Question 05 (a)
Find the ${\textstyle x}$-values of the local minima of ${\textstyle f(x)=1+12x-x^{3}}$.

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
At local minima, a function has zero derivative.

Hint 2
If a function has positive second derivative at a critical point, then this point is a local minimum.

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. We use the second derivative test, meaning that, according to Hint 1 and Hint 2, we need to find those ${\textstyle x}$-values where ${\textstyle f'(x)=0}$ and ${\textstyle f''(x)>0}$. Using the power rule, we have $$f'(x)=12-3x^{2}=-3(x^{2}-4)=-3(x+2)(x-2)$$ and $$f''(x)=-6x.$$ By the factorization of ${\textstyle f'(x)}$, the candidates for local minima are those satisfying ${\textstyle x+2=0}$ or ${\textstyle x-2=0}$, namely ${\textstyle x=-2}$ and ${\textstyle x=2}$. Testing the second derivative at these critical points, we have $$f''(-2)=-6(-2)=12>0,$$ while $$f''(2)=-6(2)=-12<0.$$ Therefore, ${\textstyle f(x)}$ attains a local minimum at ${\textstyle x=-2}$ (and a local maximum at ${\textstyle x=2}$). The only ${\textstyle x}$-value of the local minima of ${\textstyle f(x)}$ is ${\textstyle -2}$. Answer: The only ${\textstyle x}$-value of the local minima of ${\textstyle f(x)}$ is ${\textstyle {\color {blue}-2}}$.