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Elementary differential equations 7th edition - Boyce W.E

Boyce W.E Elementary differential equations 7th edition - Wiley publishing , 2001. - 1310 p.
ISBN 0-471-31999-6
Download (direct link): elementarydifferentialequat2001.pdf
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3. Compare your final solution with the one provided to see whether yours is more or less efficient than the guide, since there is frequently more than one correct way to solve a problem.
4. Ask yourself why that particular problem was assigned.
The use of a symbolic computational software package, in many cases, would greatly simplify finding the solution to a given problem, but the details given in this solutions manual are important for the student's understanding of the underlying mathematical principles and applications. In other cases, these software packages are essential for completing the given problem, as the calculations would be overwhelming using analytical techniques. In these cases, some steps or hints are given and then reference made to the use of an appropriate software package.
ssm iv
In order to simplify the text, the following abbreviations have been used:
D.E. differential equation(s)
0.D.E. ordinary differential equation(s)
P.D.E. partial differential equation(s)
1.C. initial condition(s)
I.V.P. initial value problem(s)
B.C. boundary condition(s)
B.V.P. boundary value problem(s)
I wish to express my appreciation to Mrs. Susan A. Hickey for her excellent typing and proofreading of all stages of the manuscript. Dr. Josef Torok has also provided invaluable assistance with the preparation of the figures as well as
assistance with many of the solutions involving the use of
symbolic computational software.
Charles W. Haines Professor of Mathematics and Mechanical Engineering Rochester Institute of Technology Rochester, New York June 2000
ssm v
CONTENTS
CHAPTER 1 1
CHAPTER 2 6
CHAPTER 3 35
CHAPTER 4 64
CHAPTER 5 ............................................... 73
CHAPTER 6 .............................................. 103
CHAPTER 7 121
CHAPTER 8 ............................................. 16 0
CHAPTER 9 .............................................. 174
CHAPTER 10 .............................................. 2 03
CHAPTER 11
235
CHAPTER 1
1
Section 1.1, Page 8
2. For y > 3/2 we see that y' > 0
and thus y(t) is increasing there. For y < 3/2 we have y' < 0 and thus y(t) is decreasing there. Hence y(t) diverges from 3/2 as t^ro.
4. Observing the direction field, we see that for y>-1/2 we have y'<0, so the solution is decreasing here. Likewise, for y<-1/2 we have y'>0 and thus y(t) is increasing here.
Since the slopes get closer to zero as y gets closer to -1/2, we conclude that y^-1/2 as t^ro.
If all solutions approach 3, then 3 is the equilibrium
dy dy
solution and we want ---------- < 0 for y > 3 and > 0 for
dt dt
dy
y < 3. Thus -------- = 3-y.
dt
11. For y = 0 and y = 4 we have y' = 0 and thus y = 0 and
y = 4 are equilibrium solutions. For y > 4, y' < 0 so if y(0) > 4 the solution approaches y = 4 from above. If 0 < y(0) < 4, then y' > 0 and the solutions "grow" to y = 4 as t——•. For y(0) < 0 we see that y' < 0 and the solutions diverge from 0.
13. Since y' = y2, y = 0 is the equilibrium solution and y' > 0 for all y. Thus if y(0) > 0, solutions will diverge from 0 and if y(0) < 0, solutions will aproach y = 0 as t—ro.
15a. Let q(t) be the number of grams of the substance in the
dq 300q
water at any time. Then = 300(.01)
dt 1,000,000
300(10 2- 10 6q).
/ 4
15b. The equilibrium solution occurs when q = 0, or c = 10 gm,
independent of the amount present at t = 0 (all solutions
approach the equilibrium solution).
Section 1.2
dV
16. The D.E. expressing the evaporation is ---------------- = - aS, a > 0.
dt
4 3 2
Now V = — pr and S = 4pr , so S = 4P 3
\2/3
V4PZ
V
2/3
dv
2/3
Thus --------- = - kV , for k > 0.
dt
21.
22.
24.
Section 1.2 Page 14
2
3
1b. dy/dt = -2y+5 can be rewritten as = -2dt. Thus
y-5/2
ln I y-5/2 I = -2t + c1, or y-5/2 = ce-2t. y(0) = y0 yields
c = y0 - 5/2, so y = 5/2 + (y0-5/2)e-2t.
If y0 > 5/2, the solution starts above the equilibrium solution and decreases exponentially and approaches 5/2 as t——•. Conversely, if y < 5/2, the solution starts below 5/2 and grows exponentially and approaches 5/2 from
below as t—<^>.
3
dy/dt | |
4a. Rewrite Eq.(ii) as ---------- = a and thus ln | y | = at+c1(- or
y
y = ceat.
dy dy1
4b. If y = y1(t) + k, then ------ = ----. Substituting both
dt dt
dy1
these into Eq.(i) we get = a(y1+k) - b. Since
dt
dy1
= ay1, this leaves ak - b = 0 and thus k = b/a.
dt
Hence y = y1(t) + b/a is the solution to Eq(i).
6b. From Eq.(11) we have p = 900 + cet/2. If p(0) = p0, then
c = p0 - 900 and thus p = 900 + (p0 - 900)et/2. If p0 < 900, this decreases, so if we set p = 0 and solve for T (the time of extinction) we obtain eT/2 = 900/(900-p0), or T = 2ln[900/(900-p0)].
8a. Use Eq.(26).
8b. Use Eq.(29).
dQ dQ/dt I I
10a. — = -rQ yields ---------- = -r, or ln | Q | = -rt + c1. Thus
dt Q 1
Q = ce-rt and Q(0) = 100 yields c = 100. Hence Q = 100e-rt.
Setting t = 1, we have 82.04 = 100e-r, which yields
r = .1980/wk or r = .02828/day.
dQ/dt -1
13a. Rewrite the D.E. as -------- = —, thus upon integrating and
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