Science:Math Exam Resources/Courses/MATH103/April 2013/Question 05 (b)

MATH103 April 2013

• Q1 (a) • Q1 (b) • Q1 (c) • Q1 (d) • Q1 (e) • Q1 (f) • Q2 (a) • Q2 (b) • Q2 (c) • Q2 (d) • Q3 (a) • Q3 (b) i • Q3 (b) ii • Q4 (a) • Q4 (b) • Q5 (a) • Q5 (b) • Q6 (a) • Q6 (b) • Q6 (c) • Q7 (a) • Q7 (b) • Q7 (c) • Q7 (d) • Q7 (e) • Q8 (a) • Q8 (b) • Q8 (c) • Q9 • Q10 •

Other MATH103 Exams

• April 2006 • April 2005 • April 2011 • April 2010 • April 2009 • April 2012 • April 2013 • April 2014 • April 2015 • April 2016 • April 2017 •

[hide] Question 05 (b)
Consider the differential equation $\displaystyle {\frac {dx}{dt}}=4x^{3}-12x^{2}+8x$ . Calculate the steady states (equilibria) and mark them in the sketch below. Find $\displaystyle x(t)$ for $\displaystyle t\to \infty$ for the initial points labeled a,b and c (see sketch).

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.

[show] Hint 1
Steady states are found in this case by solving for the values of x when the derivative is 0.

[show] Hint 2
At points a,b and c to see what happens as time approaches infinity, you need to know whether the derivative is positive (hence the function is increasing) or negative (hence the function is decreasing).

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.

[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. Setting the derivative to 0 and solving gives $0=4x^{3}-12x^{2}+8x=4x(x^{2}-3x+2)=4x(x-1)(x-2)$ and thus, the steady states are $x=0,1,2$ . At the point a, the derivative is positive and so the function $x(t)$ is increasing and thus approaches the steady state 1 (the first steady states bigger than a). At b, the dervative is negative hence the function is negative and thus approaches the steady state 1. At c, the derivative is positive and so the function is increasing and as t approaches infinity, the function $x(t)$ approaches infinity.