Solutions for Problems in "A First Course in Differential Equations" (11th Edition) by Dennis Zill

Chapter 1
Introduction to Differential Equations
1.1 Definitions and Terminology
1. Second order; linear
2. Third order; nonlinear because of (dy/dx)4
3. Fourth order; linear
4. Second order; nonlinear because of cos(r + u)
5. Second order; nonlinear because of (dy/dx)2 or
p
1 + (dy/dx)2
6. Second order; nonlinear because of R2
7. Third order; linear
8. Second order; nonlinear because of ẋ2
9. Writing the differential equation in the form x(dy/dx) + y2 = 1, we see that it is nonlinear
in y because of y2. However, writing it in the form (y2 − 1)(dx/dy) + x = 0, we see that it is
linear in x.
10. Writing the differential equation in the form u(dv/du)+(1+u)v = ueu we see that it is linear
in v. However, writing it in the form (v +uv −ueu)(du/dv)+u = 0, we see that it is nonlinear
in u.
11. From y = e−x/2 we obtain y′ = −1
2e−x/2. Then 2y′ + y = −e−x/2 + e−x/2 = 0.
12. From y = 6
5 − 6
5 e−20t we obtain dy/dt = 24e−20t, so that
dy
dt
+ 20y = 24e−20t
+ 20

6
5
−
6
5
e−20t

= 24.
13. From y = e3x cos 2x we obtain y′ = 3e3x cos 2x−2e3x sin 2x and y′′ = 5e3x cos 2x−12e3x sin 2x,
so that y′′ − 6y′ + 13y = 0.
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2 CHAPTER 1 INTRODUCTION TO DIFFERENTIAL EQUATIONS
14. From y = − cos x ln(sec x + tan x) we obtain y′
= −1 + sin x ln(sec x + tan x) and
y′′
= tan x + cos x ln(sec x + tan x). Then y′′ + y = tan x.
15. The domain of the function, found by solving x+2 ≥ 0, is [−2, ∞). From y′ = 1+2(x+2)−1/2
we have
(y − x)y′
= (y − x)[1 + (2(x + 2)−1/2
]
= y − x + 2(y − x)(x + 2)−1/2
= y − x + 2[x + 4(x + 2)1/2
− x](x + 2)−1/2
= y − x + 8(x + 2)1/2
(x + 2)−1/2
= y − x + 8.
An interval of definition for the solution of the differential equation is (−2, ∞) because y′ is
not defined at x = −2.
16. Since tan x is not defined for x = π/2 + nπ, n an integer, the domain of y = 5 tan 5x is
{x 5x 6= π/2 + nπ}
or {x x 6= π/10 + nπ/5}. From y′ = 25 sec2 5x we have
y′
= 25(1 + tan2
5x) = 25 + 25 tan2
5x = 25 + y2
.
An interval of definition for the solution of the differential equation is (−π/10, π/10). Another
interval is (π/10, 3π/10), and so on.
17. The domain of the function is {x 4 − x2 6= 0} or {x x 6= −2 and x 6= 2}. From
y′ = 2x/(4 − x2)2 we have
y′
= 2x

1
4 − x2
2
= 2xy2
.
An interval of definition for the solution of the differential equation is (−2, 2). Other intervals
are (−∞, −2) and (2, ∞).
18. The function is y = 1/
√
1 − sin x , whose domain is obtained from 1 − sin x 6= 0 or sin x 6= 1.
Thus, the domain is {x x 6= π/2 + 2nπ}. From y′ = −1
2(1 − sin x)−3/2(− cos x) we have
2y′
= (1 − sin x)−3/2
cos x = [(1 − sin x)−1/2
]3
cos x = y3
cos x.
An interval of definition for the solution of the differential equation is (π/2, 5π/2). Another
one is (5π/2, 9π/2), and so on.

1.1 Definitions and Terminology 3
19. Writing ln(2X − 1) − ln(X − 1) = t and differentiating
implicitly we obtain
2
2X − 1
dX
dt
−
1
X − 1
dX
dt
= 1

2
2X − 1
−
1
X − 1

dX
dt
= 1
2X − 2 − 2X + 1
(2X − 1) (X − 1)
dX
dt
= 1
dX
dt
= −(2X − 1)(X − 1) = (X − 1)(1 − 2X).
– 4 –2 2 4
t
– 4
–2
2
4
x
Exponentiating both sides of the implicit solution we obtain
2X − 1
X − 1
= et
2X − 1 = Xet
− et
(et
− 1) = (et
− 2)X
X =
et − 1
et − 2
.
Solving et − 2 = 0 we get t = ln 2. Thus, the solution is defined on (−∞, ln 2) or on (ln 2, ∞).
The graph of the solution defined on (−∞, ln 2) is dashed, and the graph of the solution
defined on (ln 2, ∞) is solid.
20. Implicitly differentiating the solution, we obtain
−2x2 dy
dx
− 4xy + 2y
dy
dx
= 0
−x2
dy − 2xy dx + y dy = 0
2xy dx + (x2
− y)dy = 0.
Using the quadratic formula to solve y2 − 2x2y − 1 = 0
for y, we get y = 2x2 ±
√
4x4 + 4

/2 = x2 ±
√
x4 + 1 .
Thus, two explicit solutions are y1 = x2 +
√
x4 + 1 and
y2 = x2 −
√
x4 + 1 . Both solutions are defined on (−∞, ∞).
The graph of y1(x) is solid and the graph of y2 is dashed.
– 4 –2 2 4
x
– 4
–2
2
4
y

21. Differentiating P = c1et/ 1 + c1et

we obtain
dP
dt
=
1 + c1et

c1et − c1et · c1et
(1 + c1et)2 =
c1et
1 + c1et

1 + c1et

− c1et

1 + c1et
=
c1et
1 + c1et

1 −
c1et
1 + c1et

= P(1 − P).
22. Differentiating y = 2x2 − 1 + c1e−2x2
we obtain
dy
dx
= 4x − 4xc1e−2x2
, so that
dy
dx
+ 4xy = 4x − 4xc1e−2x2
+ 8x3
− 4x + 4c1xe−x2
= 8x3
23. From y = c1e2x + c2xe2x we obtain
dy
dx
= (2c1 + c2)e2x
+ 2c2xe2x
and
d2y
dx2
= (4c1 + 4c2)e2x
+
4c2xe2x
, so that
d2y
dx2
− 4
dy
dx
+ 4y = (4c1 + 4c2 − 8c1 − 4c2 + 4c1)e2x
+ (4c2 − 8c2 + 4c2)xe2x
= 0.
24. From y = c1x−1 + c2x + c3x ln x + 4x2 we obtain
dy
dx
= −c1x−2
+ c2 + c3 + c3 ln x + 8x,
d2y
dx2
= 2c1x−3
+ c3x−1
+ 8,
and
d3y
dx3
= −6c1x−4
− c3x−2
,
so that
x3 d3y
dx3
+ 2x2 d2y
dx2
− x
dy
dx
+ y = (−6c1 + 4c1 + c1 + c1)x−1
+ (−c3 + 2c3 − c2 − c3 + c2)x
+ (−c3 + c3)x ln x + (16 − 8 + 4)x2
= 12x2
In Problems 25–28, we use the Product Rule and the derivative of an integral ((12) of this section):
d
dx
ˆ x
a
g(t) dt = g(x).
25. Differentiating y = e3x
ˆ x
1
e−3t
t
dt we obtain
dy
dx
= 3e3x
ˆ x
1
e−3t
t
dt +
e−3x
x
· e3x
or
dy
dx
= 3e3x
ˆ x
1
e−3t
t
dt +
1
x
, so that
x
dy
dx
− 3xy = x

3e3x
ˆ x
1
e−3t
t
dt +
1
x

− 3x

e3x
ˆ x
1
e−3t
t
dt

= 3xe3x
ˆ x
1
e−3t
t
dt + 1 − 3xe3x
ˆ x
1
e−3t
t
dt = 1

26. Differentiating y =
√
x
ˆ x
4
cos t
√
t
dt we obtain
dy
dx
=
1
2
√
x
ˆ x
4
cos t
√
t
dt +
cos x
√
x
·
√
x or
dy
dx
=
1
2
√
x
ˆ x
4
cos t
√
t
dt + cos x, so that
2x
dy
dx
− y = 2x

1
2
√
x
ˆ x
4
cos t
√
t
dt + cos x

−
√
x
ˆ x
4
cos t
√
t
dt
=
√
x
ˆ x
4
cos t
√
t
dt + 2x cos x −
√
x
ˆ x
4
cos t
√
t
dt = 2x cos x
27. Differentiating y =
5
x
+
10
x
ˆ x
1
sin t
t
dt we obtain
dy
dx
= −
5
x2
−
10
x2
ˆ x
1
sin t
t
dt +
sin x
x
·
10
x
or
dy
dx
= −
5
x2
−
10
x2
ˆ x
1
sin t
t
dt +
10 sin x
x2
, so that
x2 dy
dx
+ xy = x2

−
5
x2
−
10
x2
ˆ x
1
sin t
t
dt +
10 sin x
x2

+ x

5
x
+
10
x
ˆ x
1
sin t
t
dt

= −5 − 10
ˆ x
1
sin t
t
dt + 10 sin x + 5 + 10
ˆ x
1
sin t
t
dt = 10 sin x
28. Differentiating y = e−x2
+e−x2
ˆ x
0
et2
dt we obtain
dy
dx
= −2xe−x2
−2xe−x2
ˆ x
0
et2
dt+ex2
·e−x2
or
dy
dx
= −2xe−x2
− 2xe−x2
ˆ x
0
et2
dt + 1, so that
dy
dx
+ 2xy =

−2xe−x2
− 2xe−x2
ˆ x
0
et2
dt + 1

+ 2x

e−x2
+ e−x2
ˆ x
0
et2
dt

= −2xe−x2
− 2xe−x2
ˆ x
0
et2
dt + 1 + 2xe−x2
+ 2xe−x2
ˆ x
0
et2
dt = 1
29. From
y =
(
−x2, x 0
x2, x ≥ 0
we obtain
y′
=
(
−2x, x 0
2x, x ≥ 0
so that xy′ − 2y = 0.
30. The function y(x) is not continuous at x = 0 since lim
x→0−
y(x) = 5 and lim
x→0+
y(x) = −5. Thus,
y′(x) does not exist at x = 0.
31. Substitute the function y = emx into the equation y′ + 2y = 0 to get
(emx
)′
+ 2(emx
) = 0
memx
+ 2emx
= 0
emx
(m + 2) = 0
Now since emx 0 for all values of x, we must have m = −2 and so y = e−2x is a solution.

32. Substitute the function y = emx into the equation 5y′ − 2y = 0 to get
5(emx
)′
− 2(emx
) = 0
5memx
− 2emx
= 0
emx
(5m − 2) = 0
Now since emx 0 for all values of x, we must have m = 2/5 and so y = e2x/5 is a solution.
33. Substitute the function y = emx into the equation y′′ − 5y′ + 6y = 0 to get
(emx
)′′
− 5(emx
)′
+ 6(emx
) = 0
m2
emx
− 5memx
+ 6emx
= 0
emx
(m2
− 5m + 6) = 0
emx
(m − 2)(m − 3) = 0
Now since emx 0 for all values of x, we must have m = 2 or m = 3 therefore y = e2x and
y = e3x are solutions.
34. Substitute the function y = emx into the equation 2y′′ + 7y′ − 4y = 0 to get
2(emx
)′′
+ 7(emx
)′
− 4(emx
) = 0
2m2
emx
+ 7memx
− 4emx
= 0
emx
(2m2
+ 7m − 4) = 0
emx
(m + 4)(2m − 1) = 0
Now since emx 0 for all values of x , we must have m = −4 or m = 1/2 therefore y = e−4x
and y = ex/2 are solutions.
35. Substitute the function y = xm into the equation xy′′ + 2y′ = 0 to get
x · (xm
)′′
+ 2(xm
)′
= 0
x · m(m − 1)xm−2
+ 2mxm−1
= 0
(m2
− m)xm−1
+ 2mxm−1
= 0
xm−1
[m2
+ m] = 0
xm−1
[m(m + 1)] = 0
The last line implies that m = 0 or m = −1 therefore y = x0 = 1 and y = x−1 are solutions.

36. Substitute the function y = xm into the equation x2y′′ − 7xy′ + 15y = 0 to get
x2
· (xm
)′′
− 7x · (xm
)′
+ 15(xm
) = 0
x2
· m(m − 1)xm−2
− 7x · mxm−1
+ 15xm
= 0
(m2
− m)xm
− 7mxm
+ 15xm
= 0
xm
[m2
− 8m + 15] = 0
xm
[(m − 3)(m − 5)] = 0
The last line implies that m = 3 or m = 5 therefore y = x3 and y = x5 are solutions.
In Problems 37–40, we substitute y = c into the differential equations and use y′ = 0 and y′′ = 0
37. Solving 5c = 10 we see that y = 2 is a constant solution.
38. Solving c2 + 2c − 3 = (c + 3)(c − 1) = 0 we see that y = −3 and y = 1 are constant solutions.
39. Since 1/(c − 1) = 0 has no solutions, the differential equation has no constant solutions.
40. Solving 6c = 10 we see that y = 5/3 is a constant solution.
41. From x = e−2t + 3e6t and y = −e−2t + 5e6t we obtain
dx
dt
= −2e−2t
+ 18e6t
and
dy
dt
= 2e−2t
+ 30e6t
.
Then
x + 3y = (e−2t
+ 3e6t
) + 3(−e−2t
+ 5e6t
) = −2e−2t
+ 18e6t
=
dx
dt
and
5x + 3y = 5(e−2t
+ 3e6t
) + 3(−e−2t
+ 5e6t
) = 2e−2t
+ 30e6t
=
dy
dt
.
42. From x = cos 2t + sin 2t + 1
5et and y = − cos 2t − sin 2t − 1
5 et we obtain
dx
dt
= −2 sin 2t + 2 cos 2t +
1
5
et
and
dy
dt
= 2 sin 2t − 2 cos 2t −
1
5
et
and
d2x
dt2
= −4 cos 2t − 4 sin 2t +
1
5
et
and
d2y
dt2
= 4 cos 2t + 4 sin 2t −
1
5
et
.
Then
4y + et
= 4(− cos 2t − sin 2t −
1
5
et
) + et
= −4 cos 2t − 4 sin 2t +
1
5
et
=
d2x
dt2
and
4x − et
= 4(cos 2t + sin 2t +
1
5
et
) − et
= 4 cos 2t + 4 sin 2t −
1
5
et
=
d2y
dt2
.

43. (y′)2 + 1 = 0 has no real solutions because (y′)2 + 1 is positive for all differentiable functions
y = φ(x).
44. The only solution of (y′)2 + y2 = 0 is y = 0, since if y 6= 0, y2 0 and (y′)2 + y2 ≥ y2 0.
45. The first derivative of f(x) = ex is ex. The first derivative of f(x) = ekx is kekx. The
differential equations are y′ = y and y′ = ky, respectively.
46. Any function of the form y = cex or y = ce−x is its own second derivative. The corresponding
differential equation is y′′ −y = 0. Functions of the form y = c sin x or y = c cos x have second
derivatives that are the negatives of themselves. The differential equation is y′′ + y = 0.
47. We first note that
p
1 − y2 =
p
1 − sin2
x =
√
cos2 x = | cos x|. This prompts us to consider
values of x for which cos x 0, such as x = π. In this case
dy
dx
x=π
=
d
dx
(sin x)
x=π
= cos x x=π
= cos π = −1,
but
p
1 − y2|x=π =
p
1 − sin2
π =
√
1 = 1.
Thus, y = sin x will only be a solution of y′ =
p
1 − y2 when cos x 0. An interval of
definition is then (−π/2, π/2). Other intervals are (3π/2, 5π/2), (7π/2, 9π/2), and so on.
48. Since the first and second derivatives of sin t and cos t involve sin t and cos t, it is plausible that
a linear combination of these functions, A sin t+B cos t, could be a solution of the differential
equation. Using y′ = A cos t − B sin t and y′′ = −A sin t − B cos t and substituting into the
differential equation we get
y′′
+ 2y′
+ 4y = −A sin t − B cos t + 2A cos t − 2B sin t + 4A sin t + 4B cos t
= (3A − 2B) sin t + (2A + 3B) cos t = 5 sin t
Thus 3A − 2B = 5 and 2A + 3B = 0. Solving these simultaneous equations we find A = 15
13
and B = −10
13 . A particular solution is y = 15
13 sin t − 10
13 cos t.
49. One solution is given by the upper portion of the graph with domain approximately (0, 2.6).
The other solution is given by the lower portion of the graph, also with domain approximately
(0, 2.6).
50. One solution, with domain approximately (−∞, 1.6) is the portion of the graph in the second
quadrant together with the lower part of the graph in the first quadrant. A second solution,
with domain approximately (0, 1.6) is the upper part of the graph in the first quadrant. The
third solution, with domain (0, ∞), is the part of the graph in the fourth quadrant.

51. Differentiating (x3 + y3)/xy = 3c we obtain
xy(3x2 + 3y2y′) − (x3 + y3)(xy′ + y)
x2y2
= 0
3x3
y + 3xy3
y′
− x4
y′
− x3
y − xy3
y′
− y4
= 0
(3xy3
− x4
− xy3
)y′
= −3x3
y + x3
y + y4
y′
=
y4 − 2x3y
2xy3 − x4
=
y(y3 − 2x3)
x(2y3 − x3)
.
52. A tangent line will be vertical where y′ is undefined, or in this case, where x(2y3 − x3) = 0.
This gives x = 0 or 2y3 = x3. Substituting y3 = x3/2 into x3 + y3 = 3xy we get
x3
+
1
2
x3
= 3x

1
21/3
x

3
2
x3
=
3
21/3
x2
x3
= 22/3
x2
x2
(x − 22/3
) = 0.
Thus, there are vertical tangent lines at x = 0 and x = 22/3, or at (0, 0) and (22/3, 21/3).
Since 22/3 ≈ 1.59, the estimates of the domains in Problem 50 were close.
53. The derivatives of the functions are φ′
1(x) = −x/
√
25 − x2 and φ′
2(x) = x/
√
25 − x2, neither
of which is defined at x = ±5.
54. To determine if a solution curve passes through (0, 3) we let t = 0 and P = 3 in the equation
P = c1et/(1 + c1et). This gives 3 = c1/(1 + c1) or c1 = −3
2 . Thus, the solution curve
P =
(−3/2)et
1 − (3/2)et
=
−3et
2 − 3et
passes through the point (0, 3). Similarly, letting t = 0 and P = 1 in the equation for the
one-parameter family of solutions gives 1 = c1/(1 + c1) or c1 = 1 + c1. Since this equation
has no solution, no solution curve passes through (0, 1).
55. For the first-order differential equation integrate f(x). For the second-order differential equa-
tion integrate twice. In the latter case we get y =
´
(
´
f(x)dx) dx + c1x + c2.
56. Solving for y′ using the quadratic formula we obtain the two differential equations
y′
=
1
x

2 + 2
p
1 + 3x6

and y′
=
1
x

2 − 2
p
1 + 3x6

,
so the differential equation cannot be put in the form dy/dx = f(x, y).

57. The differential equation yy′ − xy = 0 has normal form dy/dx = x. These are not equivalent
because y = 0 is a solution of the first differential equation but not a solution of the second.
58. Differentiating we get y′ = c1 + 2c2x and y′′ = 2c2. Then c2 = y′′/2 and c1 = y′ − xy′′, so
y = y′
− xy′′

x +

y′′
2

x2
= xy′
−
1
2
x2
y′′
and the differential equation is x2y′′ − 2xy′ + 2y = 0.
59. (a) Since e−x2
is positive for all values of x, dy/dx 0 for all x, and a solution, y(x), of the
differential equation must be increasing on any interval.
(b) lim
x→−∞
dy
dx
= lim
x→−∞
e−x2
= 0 and lim
x→∞
dy
dx
= lim
x→∞
e−x2
= 0. Since dy/dx approaches 0 as
x approaches −∞ and ∞, the solution curve has horizontal asymptotes to the left and
to the right.
(c) To test concavity we consider the second derivative
d2y
dx2
=
d
dx

dy
dx

=
d
dx

e−x2

= −2xe−x2
.
Since the second derivative is positive for x 0 and negative for x 0, the solution
curve is concave up on (−∞, 0) and concave down on (0, ∞).
(d)
x
y
60. (a) The derivative of a constant solution y = c is 0, so solving 5 − c = 0 we see that c = 5
and so y = 5 is a constant solution.
(b) A solution is increasing where dy/dx = 5 − y 0 or y 5. A solution is decreasing
where dy/dx = 5 − y 0 or y 5.
61. (a) The derivative of a constant solution is 0, so solving y(a − by) = 0 we see that y = 0 and
y = a/b are constant solutions.
(b) A solution is increasing where dy/dx = y(a − by) = by(a/b − y) 0 or 0 y a/b. A
solution is decreasing where dy/dx = by(a/b − y) 0 or y 0 or y a/b.

(c) Using implicit differentiation we compute
d2y
dx2
= y(−by′
) + y′
(a − by) = y′
(a − 2by).
Thus d2y/dx2 = 0 when y = a/2b. Since d2y/dx2 0 for 0 y a/2b and d2y/dx2 0
for a/2b y a/b, the graph of y = φ(x) has a point of inflection at y = a/2b.
(d)
y = a/b
y = 0
x
y
62. (a) If y = c is a constant solution then y′ = 0, but c2 + 4 is never 0 for any real value of c.
(b) Since y′ = y2 + 4 0 for all x where a solution y = φ(x) is defined, any solution must
be increasing on any interval on which it is defined. Thus it cannot have any relative
extrema.
(c) Using implicit differentiation we compute d2y/dx2 = 2yy′ = 2y(y2 + 4). Setting
d2y/dx2 = 0 we see that y = 0 corresponds to the only possible point of inflection.
Since d2y/dx2 0 for y 0 and d2y/dx2 0 for y 0, there is a point of inflection
where y = 0.
(d)
x
y

63. In Mathematica use
Clear[y]
y[x ]:= x Exp[5x] Cos[2x]
y[x]
y''''[x] − 20y'''[x] + 158y''[x] − 580y'[x] +841y[x]//Simplify
The output will show y(x) = e5xx cos 2x, which verifies that the correct function was entered,
and 0, which verifies that this function is a solution of the differential equation.
64. In Mathematica use
Clear[y]
y[x ]:= 20Cos[5Log[x]]/x − 3Sin[5Log[x]]/x
y[x]
xˆ3 y'''[x] + 2xˆ2 y''[x] + 20x y'[x] − 78y[x]//Simplify
The output will show y(x) = 20 cos(5 ln x)/x − 3 sin(5 ln x)/x, which verifies that the correct
function was entered, and 0, which verifies that this function is a solution of the differential
equation.
1.2 Initial-Value Problems
1. Solving −1/3 = 1/(1 + c1) we get c1 = −4. The solution is y = 1/(1 − 4e−x
).
2. Solving 2 = 1/(1 + c1e) we get c1 = −(1/2)e−1. The solution is y = 2/(2 − e−(x+1)) .
3. Letting x = 2 and solving 1/3 = 1/(4 + c) we get c = −1. The solution is y = 1/(x2 − 1).
This solution is defined on the interval (1, ∞).
4. Letting x = −2 and solving 1/2 = 1/(4 + c) we get c = −2. The solution is y = 1/(x2 − 2).
This solution is defined on the interval (−∞, −
√
2 ).
5. Letting x = 0 and solving 1 = 1/c we get c = 1. The solution is y = 1/(x2 + 1). This solution
is defined on the interval (−∞, ∞).
6. Letting x = 1/2 and solving −4 = 1/(1/4 + c) we get c = −1/2. The solution is y =
1/(x2 − 1/2) = 2/(2x2 − 1). This solution is defined on the interval (−1/
√
2 , 1/
√
2 ).
In Problems 7–10, we use x = c1 cos t + c2 sin t and x′ = −c1 sin t + c2 cos t to obtain a system of
two equations in the two unknowns c1 and c2.

1.2 Initial-Value Problems 13
7. From the initial conditions we obtain the system
c1 = −1c2 = 8
The solution of the initial-value problem is x = − cos t + 8 sin t.
8. From the initial conditions we obtain the system
c2 = 0 − c1 = 1
The solution of the initial-value problem is x = − cos t.
9. From the initial conditions we obtain
√
3
2
c1 +
1
2
c2 =
1
2
−
1
2
c2 +
√
3
2
= 0
Solving, we find c1 =
√
3/4 and c2 = 1/4. The solution of the initial-value problem is
x = (
√
3/4) cos t + (1/4) sin t.
√
2
2
c1 +
√
2
2
c2 =
√
2
[6pt] −
√
2
2
c1 +
√
2
2
c2 = 2
√
2.
Solving, we find c1 = −1 and c2 = 3. The solution of the initial-value problem is x =
− cos t + 3 sin t.
In Problems 11–14, we use y = c1ex + c2e−x and y′ = c1ex − c2e−x to obtain a system of two
equations in the two unknowns c1 and c2.
c1 + c2 = 1
c1 − c2 = 2.
Solving, we find c1 = 3
2 and c2 = −1
2 . The solution of the initial-value problem is y =
3
2ex − 1
2e−x.
ec1 + e−1
c2 = 0
ec1 − e−1
c2 = e.
Solving, we find c1 = 1
2 and c2 = −1
2e2. The solution of the initial-value problem is
y =
1
2
ex
−
1
2
e2
e−x
=
1
2
ex
−
1
2
e2−x
.

e−1
c1 + ec2 = 5
e−1
c1 − ec2 = −5.
Solving, we find c1 = 0 and c2 = 5e−1. The solution of the initial-value problem is y =
5e−1e−x = 5e−1−x.
c1 + c2 = 0
c1 − c2 = 0.
Solving, we find c1 = c2 = 0. The solution of the initial-value problem is y = 0.
15. Two solutions are y = 0 and y = x3.
16. Two solutions are y = 0 and y = x2. (Also, any constant multiple of x2 is a solution.)
17. For f(x, y) = y2/3 we have
∂f
∂y
=
2
3
y−1/3
. Thus, the differential equation will have a unique
solution in any rectangular region of the plane where y 6= 0.
18. For f(x, y) =
√
xy we have ∂f/∂y = 1
2
p
x/y . Thus, the differential equation will have a
unique solution in any region where x 0 and y 0 or where x 0 and y 0.
19. For f(x, y) =
y
x
we have
∂f
∂y
=
1
x
. Thus, the differential equation will have a unique solution
in any region where x 6= 0.
20. For f(x, y) = x + y we have
∂f
∂y
= 1. Thus, the differential equation will have a unique
solution in the entire plane.
21. For f(x, y) = x2/(4 − y2) we have ∂f/∂y = 2x2y/(4 − y2)2. Thus the differential equation
will have a unique solution in any region where y −2, −2 y 2, or y 2.
22. For f(x, y) =
x2
1 + y3
we have
∂f
∂y
=
−3x2y2
(1 + y3)2 . Thus, the differential equation will have a
unique solution in any region where y 6= −1.
23. For f(x, y) =
y2
x2 + y2
we have
∂f
∂y
=
2x2y
(x2 + y2)2 . Thus, the differential equation will have a
unique solution in any region not containing (0, 0).

24. For f(x, y) = (y + x)/(y − x) we have ∂f/∂y = −2x/(y − x)2. Thus the differential equation
will have a unique solution in any region where y x or where y x.
In Problems 25–28, we identify f(x, y) =
p
y2 − 9 and ∂f/∂y = y/
p
y2 − 9. We see that f and
∂f/∂y are both continuous in the regions of the plane determined by y −3 and y 3 with no
restrictions on x.
25. Since 4 3, (1, 4) is in the region defined by y 3 and the differential equation has a unique
solution through (1, 4).
26. Since (5, 3) is not in either of the regions defined by y −3 or y 3, there is no guarantee
of a unique solution through (5, 3).
27. Since (2, −3) is not in either of the regions defined by y −3 or y 3, there is no guarantee
of a unique solution through (2, −3).
28. Since (−1, 1) is not in either of the regions defined by y −3 or y 3, there is no guarantee
of a unique solution through (−1, 1).
29. (a) A one-parameter family of solutions is y = cx. Since y′ = c, xy′ = xc = y and y(0) =
c · 0 = 0.
(b) Writing the equation in the form y′ = y/x, we see that R cannot contain any point on the
y-axis. Thus, any rectangular region disjoint from the y-axis and containing (x0, y0) will
determine an interval around x0 and a unique solution through (x0, y0). Since x0 = 0 in
part (a), we are not guaranteed a unique solution through (0, 0).
(c) The piecewise-defined function which satisfies y(0) = 0 is not a solution since it is not
differentiable at x = 0.
30. (a) Since
d
dx
tan (x + c) = sec2
(x + c) = 1+tan2
(x + c), we see that y = tan (x + c) satisfies
the differential equation.
(b) Solving y(0) = tan c = 0 we obtain c = 0 and y = tan x. Since tan x is discontinuous at
x = ±π/2, the solution is not defined on (−2, 2) because it contains ±π/2.
(c) The largest interval on which the solution can exist is (−π/2, π/2).
31. (a) Since
d
dx

−
1
x + c

=
1
(x + c)2
= y2
, we see that y = −
1
x + c
is a solution of the differ-
ential equation.
(b) Solving y(0) = −1/c = 1 we obtain c = −1 and y = 1/(1−x). Solving y(0) = −1/c = −1
we obtain c = 1 and y = −1/(1+x). Being sure to include x = 0, we see that the interval
of existence of y = 1/(1−x) is (−∞, 1), while the interval of existence of y = −1/(1+x)
is (−1, ∞).

(c) By inspection we see that y = 0 is a solution on (−∞, ∞).
32. (a) Applying y(1) = 1 to y = −1/ (x + c) gives
1 = −
1
1 + c
or 1 + c = −1
Thus c = −2 and
y = −
1
x − 2
=
1
2 − x
.
(b) Applying y(3) = −1 to y = −1/ (x + c) gives
−1 = −
1
3 + c
or 3 + c = 1.
Thus c = −2 and
y = −
1
x − 2
=
1
2 − x
.
(c) No, they are not the same solution. The interval I of definition for the solution in part
(a) is (−∞, 2); whereas the interval I of definition for the solution in part (b) is (2, ∞).
See the figure.
33. (a) Differentiating 3x2 − y2 = c we get 6x − 2yy′ = 0 or yy′ = 3x.
(b) Solving 3x2 − y2 = 3 for y we get
y = φ1(x) =
p
3(x2 − 1) , 1 x ∞,
y = φ2(x) = −
p
3(x2 − 1) , 1 x ∞,
y = φ3(x) =
p
3(x2 − 1) , −∞ x −1,
y = φ4(x) = −
p
3(x2 − 1) , −∞ x −1.
–4 –2 2 4
x
–4
–2
2
4
y
(c) Only y = φ3(x) satisfies y(−2) = 3.
34. (a) Setting x = 2 and y = −4 in 3x2 − y2 = c we get
12 − 16 = −4 = c, so the explicit solution is
y = −
p
3x2 + 4 , −∞ x ∞.
–4 –2 2 4
x
–4
–2
2
4
y

(b) Setting c = 0 we have y =
√
3x and y = −
√
3x, both defined on (−∞, ∞).
In Problems 35–38, we consider the points on the graphs with x-coordinates x0 = −1, x0 = 0,
and x0 = 1. The slopes of the tangent lines at these points are compared with the slopes given by
y′(x0) in (a) through (f).
35. The graph satisfies the conditions in (b) and (f).
36. The graph satisfies the conditions in (e).
37. The graph satisfies the conditions in (c) and (d).
38. The graph satisfies the conditions in (a).
In Problems 39–44 y = c1 cos 2x + c2 sin 2x is a two parameter family of solutions of the second-
order differential equation y′′ + 4y = 0. In some of the problems we will use the fact that
y′ = −2c1 sin 2x + 2c2 cos 2x.
39. From the boundary conditions y(0) = 0 and y
π
4

= 3 we obtain
y(0) = c1 = 0
y
π
4

= c1 cos
π
2

+ c2 sin
π
2

= c2 = 3.
Thus, c1 = 0, c2 = 3, and the solution of the boundary-value problem is y = 3 sin 2x.
40. From the boundary conditions y(0) = 0 and y(π) = 0 we obtain
y(0) = c1 = 0
y(π) = c1 = 0.
Thus, c1 = 0, c2 is unrestricted, and the solution of the boundary-value problem is y =
c2 sin 2x, where c2 is any real number.
41. From the boundary conditions y′(0) = 0 and y′ π
6

= 0 we obtain
y′
(0) = 2c2 = 0
y′
π
6

= −2c1 sin
π
3

= −
√
3 c1 = 0.
Thus, c2 = 0, c1 = 0, and the solution of the boundary-value problem is y = 0.
42. From the boundary conditions y(0) = 1 and y′(π) = 5 we obtain
y(0) = c1 = 1
y′
(π) = 2c2 = 5.
Thus, c1 = 1, c2 =
5
2
, and the solution of the boundary-value problem is y = cos 2x+
5
2
sin 2x.

43. From the boundary conditions y(0) = 0 and y(π) = 2 we obtain
y(0) = c1 = 0
y(π) = c1 = 2.
Since 0 6= 2, this is not possible and there is no solution.
44. From the boundary conditions y′ π
2

= 1 and y′(π) = 0 we obtain
y′
π
2

= 2c2 = −1
y′
(π) = 2c2 = 0.
Since 0 6= −1, this is not possible and there is no solution.
45. Integrating y′ = 8e2x + 6x we obtain
y =
ˆ
(8e2x
+ 6x) dx = 4e2x
+ 3x2
+ c.
Setting x = 0 and y = 9 we have 9 = 4 + c so c = 5 and y = 4e2x + 3x2 + 5.
46. Integrating y′′ = 12x − 2 we obtain
y′
=
ˆ
(12x − 2) dx = 6x2
− 2x + c1.
Then, integrating y′ we obtain
y =
ˆ
(6x2
− 2x + c1) dx = 2x3
− x2
+ c1x + c2.
At x = 1 the y-coordinate of the point of tangency is y = −1 + 5 = 4. This gives the initial
condition y(1) = 4. The slope of the tangent line at x = 1 is y′(1) = −1. From the initial
conditions we obtain
2 − 1 + c1 + c2 = 4 or c1 + c2 = 3
and
6 − 2 + c1 = −1 or c1 = −5.
Thus, c1 = −5 and c2 = 8, so y = 2x3 − x2 − 5x + 8.
47. When x = 0 and y = 1
2 , y′ = −1, so the only plausible solution curve is the one with negative
slope at (0, 1
2 ), or the red curve.

48. We note that the initial condition y(0) = 0,
0 =
ˆ y
0
1
√
t3 + 1
dt
is satisfied only when y = 0. For any y 0, necessarily
ˆ y
0
1
√
t3 + 1
dt 0
because the integrand is positive on the interval of integration. Then from (12) of Section 1.1
and the Chain Rule we have:
d
dx
x =
d
dx
ˆ y
0
1
√
t3 + 1
dt
1 =
1
p
y3 + 1
dy
dx
and
dy
dx
=
p
y3 + 1
y′
(0) =
dy
dx x=0
=
q
(y(0))3
+ 1 =
√
0 + 1 = 1.
Computing the second derivative, we see that:
d2y
dx2
=
d
dx
p
y3 + 1 =
3y2
2
p
y3 + 1
dy
dx
=
3y2
2
p
y3 + 1
·
p
y3 + 1 =
3
2
y2
d2y
dx2
=
3
2
y2
.
This is equivalent to 2
d2y
dx2
− 3y2
= 0.
49. If the solution is tangent to the x-axis at (x0, 0), then y′ = 0 when x = x0 and y = 0.
Substituting these values into y′ + 2y = 3x − 6 we get 0 + 0 = 3x0 − 6 or x0 = 2.
50. The theorem guarantees a unique (meaning single) solution through any point. Thus, there
cannot be two distinct solutions through any point.
51. When y = 1
16 x4, y′ = 1
4x3 = x(1
4 x2) = xy1/2, and y(2) = 1
16(16) = 1. When
y =





0, x 0
1
16
x4, x ≥ 0
we have
y′
=



0, x 0
1
4x3, x ≥ 0
= x



0, x 0
1
4x2, x ≥ 0
= xy1/2
,
and y(2) = 1
16 (16) = 1. The two different solutions are the same on the interval (0, ∞), which
is all that is required by Theorem 1.2.1.

1.3 Differential Equations as Mathematical Models
1.
dP
dt
= kP + r;
dP
dt
= kP − r
2. Let b be the rate of births and d the rate of deaths. Then b = k1P and d = k2P. Since
dP/dt = b − d, the differential equation is dP/dt = k1P − k2P.
3. Let b be the rate of births and d the rate of deaths. Then b = k1P and d = k2P2. Since
dP/dt = b − d, the differential equation is dP/dt = k1P − k2P2.
4.
dP
dt
= k1P − k2P2
− h, h 0
5. From the graph in the text we estimate T0 = 180◦ and Tm = 75◦. We observe that when
T = 85, dT/dt ≈ −1. From the differential equation we then have
k =
dT/dt
T − Tm
=
−1
85 − 75
= −0.1.
6. By inspecting the graph in the text we take Tm to be Tm(t) = 80 − 30 cos (πt/12). Then the
temperature of the body at time t is determined by the differential equation
dT
dt
= k
h
T −

80 − 30 cos
π
12
t
i
, t 0.
7. The number of students with the flu is x and the number not infected is 1000−x, so dx/dt =
kx(1000 − x).
8. By analogy, with the differential equation modeling the spread of a disease, we assume that
the rate at which the technological innovation is adopted is proportional to the number of
people who have adopted the innovation and also to the number of people, y(t), who have
not yet adopted it. If one person who has adopted the innovation is introduced into the
population, then x + y = n + 1 and
dx
dt
= kx(n + 1 − x), x(0) = 1.
9. The rate at which salt is leaving the tank is
Rout (3 gal/min) ·

A
300
lb/gal

=
A
100
lb/min.
Thus dA/dt = A/100. The initial amount is A(0) = 50.
10. The rate at which salt is entering the tank is
Rin = (3 gal/min) · (2 lb/gal) = 6 lb/min.

1.3 Differential Equations as Mathematical Models 21
Since the solution is pumped out at a slower rate, it is accumulating at the rate of (3 −
2)gal/min = 1 gal/min. After t minutes there are 300 + t gallons of brine in the tank. The
rate at which salt is leaving is
Rout = (2 gal/min) ·

A
300 + t
lb/gal

=
2A
300 + t
lb/min.
The differential equation is
dA
dt
= 6 −
2A
300 + t
.
Rin = (3 gal/min) · (2 lb/gal) = 6 lb/min.
Since the tank loses liquid at the net rate of
3 gal/min − 3.5 gal/min = −0.5 gal/min,
after t minutes the number of gallons of brine in the tank is 300 − 1
2t gallons. Thus the rate
at which salt is leaving is
Rout =

A
300 − t/2
lb/gal

· (3.5 gal/min) =
3.5A
300 − t/2
lb/min =
7A
600 − t
lb/min.
The differential equation is
dA
dt
= 6 −
7A
600 − t
or
dA
dt
+
7
600 − t
A = 6.
Rin = (cin lb/gal) · (rin gal/min) = cinrin lb/min.
Now let A(t) denote the number of pounds of salt and N(t) the number of gallons of brine
in the tank at time t. The concentration of salt in the tank as well as in the outflow is
c(t) = x(t)/N(t). But the number of gallons of brine in the tank remains steady, is increased,
or is decreased depending on whether rin = rout, rin rout, or rin rout. In any case, the
number of gallons of brine in the tank at time t is N(t) = N0 + (rin − rout)t. The output rate
of salt is then
Rout =

A
N0 + (rin − rout)t
lb/gal

· (rout gal/min) = rout
A
lb/min.
The differential equation for the amount of salt, dA/dt = Rin − Rout, is
dA
dt
= cinrin − rout
A
or
dA
dt
+
rout
A = cinrin.

13. The volume of water in the tank at time t is V = Awh. The differential equation is then
dh
dt
=
1
Aw
dV
dt
=
1
Aw

−cAh
p
2gh

= −
cAh
Aw
p
2gh .
Using Ah = π

2
12
2
=
π
36
, Aw = 102 = 100, and g = 32, this becomes
dh
dt
= −
cπ/36
100
√
64h = −
cπ
450
√
h .
14. The volume of water in the tank at time t is V = 1
3 πr2h where r is the radius of the tank
at height h. From the figure in the text we see that r/h = 8/20 so that r = 2
5h and
V = 1
3π 2
5h
2
h = 4
75 πh3. Differentiating with respect to t we have dV/dt = 4
25πh2 dh/dt or
dh
dt
=
25
4πh2
dV
dt
.
From Problem 13 we have dV/dt = −cAh
√
2gh where c = 0.6, Ah = π 2
12
2
, and g = 32.
Thus dV/dt = −2π
√
h/15 and
dh
dt
=
25
4πh2
−
2π
√
h
15
!
= −
5
6h3/2
.
15. Since i = dq/dt and L d2q/dt2 + R dq/dt = E(t), we obtain L di/dt + Ri = E(t).
16. By Kirchhoff’s second law we obtain R
dq
dt
+
1
C
q = E(t).
17. From Newton’s second law we obtain m
dv
dt
= −kv2
+ mg.
18. Since the barrel in Figure 1.3.17(b) in the text is submerged an additional y feet below
its equilibrium position the number of cubic feet in the additional submerged portion is
the volume of the circular cylinder: π×(radius)2×height or π(s/2)2y. Then we have from
Archimedes’ principle
upward force of water on barrel = weight of water displaced
= (62.4) × (volume of water displaced)
= (62.4)π(s/2)2
y = 15.6πs2
y.
It then follows from Newton’s second law that
w
g
d2y
dt2
= −15.6πs2
y or
d2y
dt2
+
15.6πs2g
w
y = 0,
where g = 32 and w is the weight of the barrel in pounds.

19. The net force acting on the mass is
F = ma = m
d2x
dt2
= −k(s + x) + mg = −kx + mg − ks.
Since the condition of equilibrium is mg = ks, the differential equation is
m
d2x
dt2
= −kx.
20. From Problem 19, without a damping force, the differential equation is m d2x/dt2 = −kx.
With a damping force proportional to velocity, the differential equation becomes
m
d2x
dt2
= −kx − β
dx
dt
or m
d2x
dt2
+ β
dx
dt
+ kx = 0.
21. As the rocket climbs (in the positive direction), it spends its amount of fuel and therefore the
mass of the fuel changes with time. The air resistance acts in the opposite direction of the
motion and the upward thrust R works in the same direction. Using Newton’s second law we
get
d
dt
(mv) = −mg − kv + R
Now because the mass is variable, we must use the product rule to expand the left side of the
equation. Doing so gives us the following:
d
dt
(mv) = −mg − kv + R
v ×
dm
dt
+ m ×
dv
dt
= −mg − kv + R
The last line is the differential equation we wanted to find.
22. (a) Since the mass of the rocket is m(t) = mp + mv + mf (t), take the time rate-of-change
and get, by straight-forward calculation,
d
dt
m(t) =
d
dt
(mp + mv + mf (t)) = 0 + 0 + m′
f (t) =
d
dt
mf (t)
Therefore the rate of change of the mass of the rocket is the same as the rate of change
of the mass of the fuel which is what we wanted to show.
(b) The fuel is decreasing at the constant rate of λ and so from part (a) we have
d
dt
m(t) =
d
dt
mf (t) = −λ
m(t) = −λt + c

Using the given condition to solve for c, m(0) = 0 + c = m0 and so m(t) = −λt + m0.
The differential equation in Problem 21 now becomes
v
dm
dt
+ m
dv
dt
+ kv = −mg + R
−λv + (−λt + m0)
dv
dt
+ kv = −mg + R
(−λt + m0)
dv
dt
+ (k − λ)v = −mg + R
dv
dt
+
k − λ
−λt + m0
v =
−mg
−λt + m0
+
R
−λt + m0
dv
dt
+
k − λ
−λt + m0
v = −g +
R
−λt + m0
(c) From part (b) we have that d
dtmf (t) = −λ and so by integrating this result we get
mf (t) = −λt+c. Now at time t = 0, mf (0) = 0+c = c therefore mf (t) = −λt+mf (0) .
At some later time tb we then have mf (tb) = −λtb +mf (0) = 0 and solving this equation
for that time we get tb = mf (0)/λ which is what we wanted to show.
23. From g = k/R2 we find k = gR2. Using a = d2r/dt2 and the fact that the positive direction
is upward we get
d2r
dt2
= −a = −
k
r2
= −
gR2
r2
or
d2r
dt2
+
gR2
r2
= 0.
24. The gravitational force on m is F = −kMrm/r2. Since Mr = 4πδr3/3 and M = 4πδR3/3 we
have Mr = r3M/R3 and
F = −k
Mrm
r2
= −k
r3Mm/R3
r2
= −k
mM
R3
r.
Now from F = ma = d2r/dt2 we have
m
d2r
dt2
= −k
mM
R3
r or
d2r
dt2
= −
kM
R3
r.
25. The differential equation is
dA
dt
= k(M − A).
26. The differential equation is
dA
dt
= k1(M − A) − k2A.
27. The differential equation is x′(t) = r − kx(t) where k 0.

28. Consider the right triangle fromed by the waterskier (P),
the boat (B), and the point on the x-axis directly below
the waterskier. Using Pythagorean Theorem we have that
the base of the triangle on the x-axis has length
p
a2 − y2.
Therefore the slope of the line tangent to curve C is
y′
= −
y
a2 − y2
Notice that the sign of the derivative is negative because as the boat proceeds along the
positive x-axis, the y-coordinate decreases.
29. We see from the figure that 2θ + α = π. Thus
y
−x
= tan α = tan(π − 2θ) = − tan 2θ = −
2 tan θ
1 − tan2 θ
.
Since the slope of the tangent line is y′ = tan θ we have
y/x = 2y′[1−(y′)2] or y−y(y′)2 = 2xy′, which is the quadratic
equation y(y′)2 + 2xy′ − y = 0 in y′. Using the quadratic
formula, we get
y′
=
−2x ±
p
4x2 + 4y2
2y
=
−x ±
p
x2 + y2
y
.
(x, y)
x
y
α
α
θ
θ
θ
φ
x
y
Since dy/dx 0, the differential equation is
dy
dx
=
−x +
p
x2 + y2
y
or y
dy
dx
−
p
x2 + y2 + x = 0.
30. The differential equation is dP/dt = kP, so from Problem 41 in Exercises 1.1, a one-parameter
family of solutions is P = cekt.
31. The differential equation in (3) is dT/dt = k(T − Tm). When the body is cooling, T Tm,
so T − Tm 0. Since T is decreasing, dT/dt 0 and k 0. When the body is warming,
T Tm, so T − Tm 0. Since T is increasing, dT/dt 0 and k 0.
32. The differential equation in (8) is dA/dt = 6 − A/100. If A(t) attains a maximum, then
dA/dt = 0 at this time and A = 600. If A(t) continues to increase without reaching a
maximum, then A′(t) 0 for t 0 and A cannot exceed 600. In this case, if A′(t) approaches
0 as t increases to infinity, we see that A(t) approaches 600 as t increases to infinity.
33. This differential equation could describe a population that undergoes periodic fluctuations.

Solutions for Problems in "A First Course in Differential Equations" (11th Edition) by Dennis Zill

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Solutions for Problems in "A First Course in Differential Equations" (11th Edition) by Dennis Zill