Science:Math Exam Resources/Courses/MATH105/April 2013/Question 04 (a)

MATH105 April 2013

• Q1 (a) • Q1 (b) • Q1 (c) • Q1 (d) • Q1 (e) • Q1 (f) • Q1 (g) • Q1 (h) • Q1 (i) • Q1 (j) • Q1 (k) • Q1 (l) • Q1 (m) • Q1 (n) • Q2 (a) • Q2 (b) • Q2 (c) • Q3 (a) • Q3 (b) • Q4 (a) • Q4 (b) • Q5 (a) • Q5 (b) • Q6 (a) • Q6 (b) •

Other MATH105 Exams

• April 2009 • April 2011 • April 2010 • April 2012 • April 2013 • April 2014 • April 2015 • April 2016 • April 2017 • April 2018 •

[hide] Question 04 (a)
Let $\displaystyle f(x,y)=xye^{-2x-y}$ . Find all critical points of $f(x,y)$ .

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!

[show] Hint
What is true about the partial derivatives of a multi-variable function at critical points?

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.

[show] 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. The critical points of a multivariable function $f(x,y)$ are the values of $(x,y)$ where the following conditions are both simultaneously true: ${\begin{cases}{\frac {\partial f}{\partial x}}(x,y)=0\\{\frac {\partial f}{\partial y}}(x,y)=0\end{cases}}$ Since $\displaystyle f(x,y)=xye^{-2x-y}$ , the above conditions become: ${\begin{cases}ye^{-2x-y}-2xye^{-2x-y}=0\\xe^{-2x-y}-xye^{-2x-y}=0\end{cases}}$ To solve these equations, we begin by factoring the left-hand sides, giving us ${\begin{cases}y(1-2x)e^{-2x-y}=0\\x(1-y)e^{-2x-y}=0\end{cases}}$ Since the exponential function is never zero, the equations above are equivalent to ${\begin{cases}y(1-2x)=0\quad &(1)\\x(1-y)=0.\quad &(2)\end{cases}}$ We now proceed to determine the values of $(x,y)$ such both Eq. (1) and (2) hold. The solutions to Eq. (1) are $y=0$ and $x=1/2$ . We take each of these solutions and plug them into Eq. (2). If $y=0$ , then Eq. (2) becomes: $x(1-0)=0\quad \rightarrow \quad x=0.$ Hence one of the critical points is $(x,y)=(0,0)$ . If $x=1/2$ , then Eq. (2) becomes: ${\frac {1}{2}}(1-y)=0\quad \rightarrow \quad y=1.$ So, another critical point is $(x,y)=(1/2,1)$ . Therefore, the critical points of $f$ are ${\color {blue}\displaystyle (x,y)=(0,0),\left({\frac {1}{2}},1\right).}$