Course:MATH102/Question Challenge/2001 December Q7
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{{#w4grb_rate:}} Hard Easy |
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Question
Solutions to the differential equation
starting at three different initial values are shown in the graph. Also shown are the tangent lines to these curves at
You are given the following information about the slopes of these tangent lines:
(i) The slope is five times the slope .
(ii) The slope is 3.
Use this information to answer the following questions:
(a) Determine the values of the constants a and b. (Justify your answer).
(b) Determine the value that will approach after a long time on any of these curves.
(c) Find the value of at time given that . Justify how your answer was obtained.
Hints
Hint 1: |
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What is the differential equation saying about the slope of a tangent line to each solution curve? How is that slope related to the current value of ? |
Hint 2: |
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Look at the values of at t=0 on each of the curves and relate to and . |
Hint 3: |
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You should get some equations for . Solve those equations to find the values of these constants |
Hint 4: |
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blah |
Solutions
Solution |
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Part A) The slopes have to match with the value of when respectively, which provides a number of equations. We know that:
We also know:
we are given that and Therefore by extension: and The first of these can also be written as after simplification. We now have two equations in two unknowns, .
We can solve these to obtain
Part B) To find the value will approach after a very long time on any of these curves we must find a stable steady state. First we will set :
Solving for :
A phase graph would show that for values of the rate of change . These values will move to the right towards . For values of the rate of change . These values will moves to the left towards . This describes a stable steady state so these curves will approach this value of over time. Part C) Solution to differential equation of the form are given by:
Solving for :
Comment: A solution using Euler's method was also acceptable and would have resulted in a solution of 1.54 |