Science:Math Exam Resources/Courses/MATH110/April 2019/Question 05 (a)

MATH110 April 2019

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Question 05 (a)
Let $f(x)={\frac {x^{3}}{x^{2}+1}}$. Find the critical point(s) of f(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
The critical points of a function f(x) are the points in its domain at which f ′(x) = 0. To compute these, you can use the quotient rule.

Hint 2
Compute the derivative of the function using the quotient rule: $f'(x)={\frac {(x^{3})'(x^{2}+1)-x^{3}(x^{2}+1)'}{(x^{2}+1)^{2}}}.$ Then, use that a function of the form ${\frac {g(x)}{h(x)}}$ is zero if and only if its numerator $g(x)$ is zero and its denominator $h(x)$ is nonzero.

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. Recall that the critical points of a function f(x) are the points in its domain where f ′(x) is zero. First, compute the derivative of the function using the quotient rule: $f'(x)={\frac {(x^{3})'(x^{2}+1)-x^{3}(x^{2}+1)'}{(x^{2}+1)^{2}}}={\frac {(3x^{2})(x^{2}+1)-x^{3}(2x)}{(x^{2}+1)^{2}}}.$ After expanding all the terms in the numerator and simplifying, the result is: $f'(x)={\frac {x^{4}+3x^{2}}{(x^{2}+1)^{2}}}={\frac {x^{2}(x^{2}+3)}{(x^{2}+1)^{2}}}.$ The critical points are the solutions to the equation f ′(x)=0. Note that the denominator of f'(x) is strictly bigger than 0 since $x^{2}\geq 0$, so $x^{2}+1\geq 1$ (and 1 > 0). By hint 2, in this case, f'(x) = 0 is equivalent to $x^{2}(x^{2}+3)=0$. To solve this, note that the equation is satisfied when $x^{2}=0$ or $x^{2}+3=0$. The first equation has solution $x=0$; the second has no solution because $x^{2}+3\geq 3>0$ for all $x$. Therefore, the only critical point is $x=0$.