306FF
TEST #2 - Study Guide
Problem #1:
See section 2.1 in Text #1
A Predator Prey Model
(a)-[5 pts.]- A population of wolves and mice exclusively cohabit the same isolated island.
Translate the following statements into a 1st order system of differential equations:
(i) In the absence of the wolves the mouse population would increase exponentially.
(ii) In the absence of the mice, the wolf population would decrease exponentially.
(iii) Interactions between wolves and mice are favorable to the wolves; the rate of growth of
the wolves is increased by an amount jointly proportional to the wolf and mouse population.
Interactions between wolves and mice are unfavorable for mice; the rate of growth of the
mouse population is decreased by an amount jointly proportional to the wolf and mouse
population.
(b)-[5 pts.]- Sketch a phase portrait for the predator-prey system you obtained in part (a),
where W(t) stands for the Wolf population at time t  0, and M(t) stands for the mouse population at time t  0.
Problem# 1 (continued)
A Competing Species Model
See section 2.2 in Text #1.
(c)-[4 pts.]- A population of sheep and cows exclusively cohabit the same isolated island. Translate the following
statements into a 1st order system of differential equations:
(i) In the absence of the cows the sheep population would increase logistically.
(ii) In the absence of the sheep the cow population would increase logistically.
(iii) Since the cows and sheep feed on the same grassland the interaction between
cows and sheep is unfavorable to both species. The rate of growth of the cow
population is decreased by an amount jointly proportional to the cow and sheep
population. The rate of growth of the sheep population is decreased by an amount
jointly proportional to the cow and sheep population.
(d)-(i)-[3 pts.]- Sketch the solution of an initial value problem for the competing species system you obtained in
part (c), where S(t) stands for the Sheep population at time t  0, and C(t) stands for the cow population at time
t  0.
(d)-(ii)-[3 pts.]- Sketch the solution of an initial value problem for the competing species system you obtained in
part (c), where S(t) stands for the Sheep population at time t  0, and C(t) stands for the cow population at time
t  0.
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Problem # 2: Analytic Methods for Special Systems: An initial value problem for a non-linear,
1st order system of o.d.e.'s
See section 2.3 in Text #1
(a)-[15 pts]- Calculate the exact solution [x(t), y(t)] of the following
initial value problem:
x' = f(x), y' = a(x)y + g(x), x(0) = x0, y(0) = y0.
(b)-[5 pts]- Sketch the graphs of x(t) and y(t).
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Problem # 3: Analytic Methods for Special Systems: An initial value problem for a
non-linear, 1st order system of o.d.e.'s
See section 2.3 in Text #1
(a)-[15 pts.]- Solve the following initial value problem:
x' = f(y) x, y' = g(y), x(0) = x0, y(0) = y0 .
(b)-[5 pts.]- Sketch the graph of [x(t), y(t)].
CONTINUED ON NEXT PAGE
C. O. Bloom
306FF
TEST #2 - Study Guide
Problem # 4:
Euler's Method
See section # 2.4 of Text #1
(a)-[15 pts.]- Use Euler's method with step size ∆t = # to approximate the solution
of the following initial value problem: x' = f(x, y) , y' = g(x, y), x(0) = x0, y(0) = y0
over the interval a  t  b .
(b)-[5 pts.]- Plot the points (xi, yi ) found by Euler's method and draw a polygonal arc
through these points.
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Problem #5:
Linear Systems
See problems 26-29 in section 3.1 of Text #1.
v
v
(a)-[5 pts.] -Verify that Y (t)  e 1t  11 and Y (t)  e  2 t  12  are solutions of Y '  AY .
1
2


v21 

v22 

(b)-[5 pts.]-Verify that Y1(0) and Y2 (0) are linearly independent, i.e. set k1Y1 (0)  k2Y2 (0)  0
and show that k1  0 and k 2  0 .
(c)-[5 pts.]-Write down the general solution of Y '  AY .
x 
(d)-[5 pts.]-Find k1 and k2 such that Y(t)  k1Y1(t) + k2Y2(t) satisfies the initial condition Y (0)   0  .
 y0 
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Problem # 6:
An Overdamped Harmonic Oscillator Problem
See section 3.1, problem 16. See also “A Harmonic Oscillator” on page 261 and
“An Overdamped Harmonic Oscillator” on page 325. See section 3.6 pp. 316-321.
Do problems (3.6) #1, 5, 7, 9, 11.
(a)-[5 pts.]- Re-write the differential equation ay ' 'by ' cy  0 as an equivalent 1st
order system of two linear o.d.e’s in two unknowns. Here a, b and c are positive constants.
(b)-[5 pts.]- Write the system of part (a) in matrix-vector notation, i.e. in the form
Y '  AY where A is a 2x2 matrix and Y is a 2x1 vector.
(c)-[5 pts.]- Find the general solution of the system found in part (b).
(d)-[5 pts.]- Use the result of (c) to find the general solution of ay ' 'by ' cy  0 .
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