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Math 333 Test One Solutions and Instructions, Exams of Differential Equations

Instructions and problems for a math 333 university exam. It includes various types of differential equations and initial value problems, requiring students to classify orders, linearity, and check given solutions. Additionally, students are asked to sketch slope fields, analyze equilibrium solutions, and solve separable and linear inhomogeneous equations. Some problems involve change of variables and numerical methods.

Typology: Exams

Pre 2010

Uploaded on 08/19/2009

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Math 333 Test One
Dr. Holmes
February 7, 2002
This exam will start at 7:40 am and end at 8:35 am.
No access to books, notes or neighbors is permitted. You are permitted
to use a scientific calculator with no symbolic or graphing capabilities (this
was announced in class).
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Download Math 333 Test One Solutions and Instructions and more Exams Differential Equations in PDF only on Docsity!

Math 333 Test One

Dr. Holmes

February 7, 2002

This exam will start at 7:40 am and end at 8:35 am. No access to books, notes or neighbors is permitted. You are permitted to use a scientific calculator with no symbolic or graphing capabilities (this was announced in class).

  1. Classify the following differential equations and initial value problems: for each equation state its order and whether it is linear or nonlinear. In the second part, a “solution” is given (a function or family of func- tions). Check whether the given family of functions is actually a family of solutions of the equation: I may not be telling the truth! Show all work for your check.

(a) x′^ = sec(t)x

(b) x′′^ = (x′)^2 purported solution: x(t) = − ln(t + c)

  1. Solve the linear inhomogeneous initial value problem.

(a) x′^ = 2x − t; x(0) = 1

(b) (extra credit) Solve the linear inhomogeneous equation

v′^ = tan(t)v + cos(t)

  1. We analyze the differential equation

x′^ = (x − 2)(x − 4).

Sketch the slope field for this equation. Using the slope field, sketch several solutions, representing the different kinds of qualitative behavior that solutions to this equation can have. Your sketch should be good enough that I can tell what is happening

  • it does not have to rival Maple for accuracy! This equation is autonomous. Identify the equilibrium solutions and draw the phase line. Identify each equilibrium solution as a source, sink, or node, showing all supporting calculations.
  1. An initial value problem is given:

x′^ = x

1 (^3) + x 2 (^3) ; x(0) = 0

Tell whether existence of a solution follows from theorems we have studied (this is very different from asking whether you can solve the equation!). If you conclude that there is a solution, tell me whether the existence of just one solution (uniqueness) follows from the theorems we have studied. Explain why, stating clearly what relevant conditions in those theorems hold or do not hold in each case. Please note that you are not being asked to solve this equation.

  1. Numerical methods. You have the choice of doing the theoretical part a or the practical part b (next page).

(a) For what initial value problems

x′^ = f (x, t); x(t 0 ) = x 0

does Euler’s method always give precisely correct answers for any x(t 1 )? Your answer should include a precise description of what kind of function f (t, x) can be, and some kind of supporting ar- gument (your “supporting argument” can be pictorial).

  1. A tank of water has a capacity of 300 liters. We start with the tank one- third full. We add a 15 percent salt solution at 3 liters/minute while draining the mixture in the tank at 2 liters/minute at the same time. We assume (unrealistically) that the salt is dispersed evenly throughout the mixture as soon as it is added. How much salt is there in the tank at the moment it fills? Set up the initial value problem to be solved, and explain how you would use the solution to the initial value problem to answer the question. You are not required to actually solve the initial value problem or compute the final answer.